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East-West Orientation of Historical Empires

Peter Turchin, Jonathan M. Adams, and Thomas D. Hall

November 2004

Does environment affect the ability of states to project power? If state expansion is more easily
accomplished by staying within the same ecological zone, then state territories should be oriented
in the east-west direction, mirroring the orientation of major ecological zones of the world. Our
analysis of 62 largest empires in history supports this conjecture.

In a chapter entitled “Spacious Skies and Tilted Axes” Jared Diamond (1997) argued that

food production spreads more easily latitudinally (in the East-West direction) rather than
longitudinally. A latitudinal shift is easier because similar climates and soil types tend to be
arranged in east-west oriented bands. This geographic pattern is best illustrated by a map of the
global distribution of biomes (Figure 1). A

biome

is a major type of ecological community such

as the grassland, desert, or temperate seasonal forest (Ricklefs 2001).

Although Diamond focused primarily on the spread of crop cultivars and domesticated

animals, the same principle should influence the military/political, demographic, and cultural
dynamics of societies. Consider the greatest empire ever (in terms of territorial extent), conquered
by the Mongols under Chinggis Khan and his immediate successors. The core of the Mongolian
Empire was the Great Eurasian Steppe that stretches for many thousands of kilometers from the
Khingan Mountains in the east to the Carpathians in the west (McNeill 1964). The Mongols were
steppe warriors, and they were able to rapidly extend their influence over this whole region
(Barfield 1994). The regions inhabited by settled agriculturalists adjacent to the steppe were
incorporated more slowly and to a lesser degree than the steppe. For example, the Russian
principalities of the forest zone were not occupied by the steppe-dwellers, being instead subjected
to tribute. As a result, the Mongolian Empire, with its core based on the steppe, was much wider
in the latitudinal rather than longitudinal direction. The ease of conquest was not the only factor
promoting the latitudinal spread of the great empires. Societies inhabiting similar ecological
zones tend to be more similar to each other than societies located in very different zones.
Techniques developed for integrating and controlling a certain type of society should, therefore,
be easier to extend latitudinally. Note that this “ecologic factor” should be detectable only at large
geographic scales—a small state, as long as it stays within the same biome, will find it equally
easy (or equally hard) to expand in any direction, longitudinal or latitudinal.

Territorial expansion by states is a complex macrosociological process, influenced by

many factors other than the environment. Does the ecological factor have a detectable effect on
the projection of military/political power, or is its influence lost in the “noise” of complex
interactions? To answer this question, we compiled a list of all large historical empires with peak
territories exceeding 1 Mm

(= 1,000,000 km

), and measured the distances from their eastern to

western extremes, as well as from the northern to southern extreme (Endnote 1). Our measure of
the tendency to expand in the latitudinal direction is the log-transformed ratio of the east-west
distance to north-south distance (Endnote 2) the “latitudinal index.”

The frequency distribution of the latitudinal index in our sample of 62 historic empires is

strongly skewed to the right (Figure 2), and the mean index is significantly greater than zero (

4.83,

< 0.001). The great majority of empires, nearly 80%, have a positive latitidinal index—

that is, they are wider in the east-west compared to the north-south direction. There are only three
empires that have a strong north-south orientation, and these are the proverbial exceptions that
prove the rule. The New Kingdom of Egypt had at its core the valley of a major river running
south-north, the Nile. The Inca empire was located on the west coast of South America, where

ecological zones run longitudinally (see Figure 1) due to a major mountain chain, the Andes.
Finally, the Khmer empire was located entirely within the wet tropical forest biome. Thus, even
though these three cases do not conform to the rule of latitudinal spread, they obey the more
general rule of “expansion within an ecological zone.”

All of the largest empires (with territory over 10 Mm

) were oriented in the east-west

direction. We have already discussed the case of the Mongol empire. The Islamic Caliphate is a
variation on the same pattern, except that the “native biome” of the Arabs was the subtropical
desert, rather than the temperate grassland/desert of the Mongols. The next largest state in history
after the Mongols, the Russian empire (peak area of 22.8 Mm

in 1895), originated in the

transitional zone between the steppe and the forest (ecologists call such transitional zones

ecotones

). Once the Muscovite state began expanding in the sixteenth century, it spread fastest

precisely within the same ecotone—eastward along the boundary between the Eurasian steppe
and northern taiga. Eastward expansion was extremely rapid, so that the Pacific was reached by
the mid-seventeenth century. In contrast, the southern advance into the steppes and deserts of
Central Asia took a much longer time, and they were conquered only by the late nineteenth
century.

Another example of the same dynamic is the early expansion of Rome. The territory of

the Roman Empire in the first century B.C.E. coincides almost precisely with the
woodland/shrubland biome (also known as the Mediterranean zone). Subsequent expansion took
the Romans into the forests of northern Europe. However, severe reverses, such as the battle of
Teutoburg Forest in 9 C.E., in which 20,000 legionnaires were obliterated by the tribal Germans
(Wells 2003), persuaded the Romans to abandon plans of further conquest. The general rule, thus,
seems to be that expansion is easiest and most lasting when occuring within the same ecological
zone. Expansion into other biomes is possible, but more difficult, slow, and requires greater state
resources. The case of China is probably the best illustration of this principle. The native biome
of the Chinese is the temperate seasonal forest (Endnote 3), and this was precisely the area that
was first unified by each of a long succession of Chinese empires. The strength of the Chinese
state, however, allowed it to expand into alien biomes, and at their peaks the great Chinese
empires intruded into the steppe (Inner Mongolia, Chinese Turkestan), the alpine biome (Tibet),
and the tropical rain forest (Vietnam).

Is the influence of ecology detectable in the shapes of modern states? At first glance, no.

The average latitude index for the 29 modern states whose territory exceeds 1 Mm

is positive,

but not significantly different from 0. However, if we exclude South American countries, where
biomes extend in the longitudinal direction, the statistical test indicates that the pattern is
detectable even today (

= 2.66,

= 0.014). The tendency to east-west orientation in modern

countries, nevertheless, is much weaker than for historical empires. Partly this could be due to the
effect of modern technology, but we suspect that it is also an artefact of the propensity of modern
states to claim territory even if it is not used by their populations. A striking example of this
tendency is Canada, whose population is squeezed into a narrow band running east-west along its
southern border with the US, but which nevertheless claims extensive territories in the Arctic.
Because of the addition of these lands, which are almost totally devoid of human occupation, the
latitudinal index of Canada is slightly negative. Algeria and Lybia provide other examples of the
same tendency—their populations are largely confined to the east-west band along the
Mediterranean littoral, but their latitudinal indices are essentially zero, because they claim huge
territories to the south, in the Saharan desert.

In conclusion, our results indicate that the physical and biological environment has a

detectable effect on the shapes of historic and modern states. It appears that projection of

military/political power is easier within the same ecological zone (biome). Our results should not
be interpreted as a kind of “ecological determinism”—although ecology is important, its
influence on state expansion patterns is transmitted by entirely social mechanisms. On the other
hand, certain techniques and ideas from ecological sciences have proved to be fruitful in
suggesting novel approaches to the study of social systems (Turchin and Hall 2003). Diamond’s
original insight, which motivated our study, is one example; another is the recent demonstration
that cultural variability exhibits a latitudinal gradient (Pagel and Mace 2004). Our results also
have interesting implications for the study of historical dynamics (Turchin 2003). Researchers
working within the world-system paradigm have noted that the rise and fall of populations, cities,
and empires is characterized by a broad-scale synchronicity (Chase-Dunn and Hall 1997, Chase-
Dunn et al. 2000). For example, there is a substantial correlation between the dynamics of
Western Europe and China. On the other hand, South Asian dynamics are completely
uncorrelated with the rest of Eurasia. Our finding that the propagation of “signals” within
military-political networks is facilitated in the latitudinal, but not longitudinal directions suggest
one possible explanation for this macrohistoric pattern.

Endnotes:

Note 1. Our list of large historical states was based on the compilation by Taagepera (1978, 1978,
1979, 1997), which has been systematized and posted on the web by Chase-Dunn and coworkers
(http://www.irows.ucr.edu/). We checked the Taagepera list with all major historical atlases in the
library of the University of Connecticut and found additional eight empires that fitted our criteria
(Axum, Hsi-Hsia, Kara-Khitai, Srivijaya, Maurian, Kushan, Gupta, and Maratha). We excluded
the maritime empires of the European Great Powers, because our measure of the latitudinal
tendency is not applicable to such noncontiguous, widely distributed collections of territories.
One difficulty in constructing the list was presented by the repeated rise of empires in the same
location, such as in China. We adopted the middle road of counting each major dynasty (Han,
Tang, Ming, etc) as a separate empire, but did not distinguish between cycles within a dynasty
(e.g., Early versus Late Han). Analysis of a reduced dataset, which included only the largest
empire for each geographic location, yielded qualitatively the same result. See Table 1 for the list
of empires.

Note 2. Log-transforming the ratio of distances was necessary to make the distribution of the
index symmetric. Positive values indicate east-west orientation, and negative values – north-south
orientation.

Note 3. It may seem strange to call the Chinese home biome a “forest,” because in present-day
China, of course, very few forests are left. Remember, however, that the biome names reflect the
types of ecological communities that would be present before substantial human impact; the
names are simply a short-hand reference to particular combinations of the climate and soil types.

References

Barfield, T. J. 1994. The devil's horsemen. Pages 157-182

S. P. Reyna and R. E. Downs, editors.

Studying war: anthropological perspectives. Gordon and Breach, Langhorn, PA.

Chase-Dunn, C., S. Manning, and T. D. Hall. 2000. Rise and fall: east-west synchronicity and Indic

exceptionalism reexamined. Social Science History

:727-754.

Chase-Dunn, C. K., and T. D. Hall. 1997. Rise and demise: comparing world-systems. Westview Press,

Boulder, CO.

Diamond, J. 1997. Guns, Germs, and Steel: The Fates of Human Societies. W. W. Norton, New York.
McNeill, W. H. 1964. Europe's Steppe Frontier. University of Chicago Press, Chicago, IL.
Pagel, M., and R. Mace. 2004. The cultural wealth of nations. Nature

428

:275-278.

Ricklefs, R. E. 2001. The Economy of Nature, 5th edition. W. H. Freeman, New York.

Taagepera, R. 1978. Size and duration of empires: systematics of size. Social Science Research

:108-127.

Taagepera, R. 1978. Size and duration of empires; growth-decline curves, 3000 to 600 BC. Social Science

Research

:180-196.

Taagepera, R. 1979. Size and duration of empires: growth-decline curves, 600 B.C. to 600 A.D. Social

Science History

:115-138.

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Studies Quarterly

:475-504.

Turchin, P. 2003. Historical dynamics: why states rise and fall. Princeton University Press, Princeton, NJ.
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theoretical ecology. Journal of World Systems Research

:37-64.

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Legions in the Teutobutg Forest. W. W. Norton, New York.

Table 1.

The large historical states used in the analysis.

Date

(peak)

Empire name

World

Region

Area

(Mm

)

Latitude

Index

-1300 Egypt (New Kingdom) Africa

1.00

-1.292

350 Axum

Africa

1.25

0.241

969 Fatimid

Africa

4.10

0.782

1120 Almoravid

Africa

1.00

0.561

1200 Almohad

Africa

2.00

0.864

1380 Mali

Africa

1.10

0.512

1400 Mameluk

Africa

2.10

-0.225

1527 Inca

America

2.00

-1.139

-176 Hsiung-Nu (Hunnu)

Central Asia

9.00

0.818

405 Juan-Juan

Central

Asia

2.80

0.740

557 Turks

Central

Asia

6.00

1.026

800 Uigur

Central

Asia

3.10

0.213

800 Tufan (Tibet)

Central Asia

4.60

0.605

850 Khazar

Central

Asia

3.00

0.139

1100 Hsi-Hsia

Central

Asia

1.00

0.655

1210 Khorezm

Central

Asia

2.30

0.054

1210 Kara-Khitai

Central

Asia

1.50

0.362

1270 Mongol

Central

Asia

24.00

0.737

1310 Golden Horde

Central Asia

6.00

0.153

1350 Chagatai

Central

Asia

3.50

0.383

1405 Timur’s

Central

Asia

4.40

0.426

-1122 Shang

East

Asia

1.25

0.050

-50 China-Early

Han

East

Asia

6.00

0.661

579 Liang

East

Asia

1.30

0.137

715 China-Tang

East

Asia

5.40

0.375

947 Liao (Kitan)

East Asia

2.60

0.606

980 China-Sung

East

Asia

3.10

-0.164

1126 Jurchen (Chin)

East Asia

2.30

-0.147

1450 China-Ming

East

Asia

6.50

-0.138

1790 China-Manchu

East

Asia

14.70

0.246

117 Rome

Europe

5.00

0.204

441 Huns

(Atilla’s)

Europe

4.00

1.003

555

East

Roman Europe 2.70

0.516

814 Frankish

Europe

1.20

0.092

1000 Kiev

Europe

2.10

-0.132

1025 Byzantine

Europe

1.35

0.806

1480 Lithuania-Poland

Europe

1.10

0.079

1683 Ottoman

Europe

5.20

0.320

1895 Russia

Europe

22.80

0.303

1200 Srivijaya

Southeast

Asia

1.20

0.272

1290 Khmer

Southeast

Asia

1.00

-0.665

-250 Mauryan

South

Asia

5.00

0.191

200 Kushan

South

Asia

2.00

0.095

400 Gupta

South

Asia

3.50

-0.031

648 Harsha

(Kanyakubia) South Asia

1.00

0.668

1312 Delhi

South

Asia

3.20

-0.082

1690 Mughal

South

Asia

4.00

0.435

1760 Maratha

South

Asia

2.50

-0.280

-670 Assyria

Southwest

Asia

1.40

1.845

-585 Media

Southwest

Asia

2.80

0.141

-500 Achaemenid Persia

Southwest Asia

5.50

0.200

-323 Alexander’s

Southwest

Asia

5.20

0.478

-301 Seleucid

Southwest

Asia

3.90

0.882

0 Parthia

Southwest

Asia

2.80

1.374

550 Sassanian Persia

Southwest Asia

3.50

0.292

750 Caliphate

Southwest

Asia

11.10

0.730

928 Samanid

Southwest

Asia

2.85

-0.194

980 Buyid (Buwahid)

Southwest Asia

1.60

0.142

1029 Ghaznavid

Southwest

Asia

3.40

0.689

1080 Seljuk

Southwest

Asia

3.90

0.409

1190 Ayyubids

Southwest

Asia

2.00

-0.300

1310 Il-Khan

Southwest

Asia

3.75

0.664

Table 2

Region Mean

Africa 0.21

0.28

0.72

0.50

Central

Asia

0.48 0.08 13 5.80 <0.001

East Asia

0.18 0.10 9 1.71

0.12

Europe 0.35

0.12

2.92

0.02

South & SE Asia

0.07 0.13 9 0.51

0.62

Southwest

Asia

0.52 0.15 14 3.40

0.005

America

–1.14 – 1 – –

All

regions

0.31 0.06 62 4.83 <0.001