Nationalism threw unrelated peoples together as ex-colonies became newly created states. The borders of many nations were drawn by the Great Powers at Versailles, at the end of World War One. Many borders were wholly artificial--some were even meant to divide people of one tribe. The Pashtun, who've been ungovernable ever since Alexander conquered them, inhabit southern Afghanistan and parts of Pakistan. The English, seeing the threat of a unified Pashtun nation, split the border of the two countries so the Pashtun would be divided and theoretically less of a threat to either nation. Now we see the border as little more as an insignificant line between people who share common ethnicity, language, and religion.
The greatest threats to empire emerge not from other empires but from nationalization--the process of people in one region unifying and throwing out all their colonial masters. Marxism, with its liberation theologies, became an effective tool in the expansion of communism.. As people cast off their occupiers, they become susceptible to charismatic leaders like Castro, and Nasser of Egypt, who can unify resistance to whatever interests that the empire might have left behind and continue to want to protect.
In some ways, we are still contending with the strong reaction among Arabs to the Balfour Declaration and Anglo-French Agreement, which formed the basis of a post-war order between France, England, and the United States. Syria and Lebanon were assigned to the French sphere of influence, TransJordan (including Palestine) and most of Iraq went to the British, with the US getting Saudi Arabia and its newly discovered oil fields.
Whatever problems the Middle East had after World War I were insignificant compared to the creation of Israel in 1948, which imposed arbitrary borders on land previously occupied by Palestinians. At least six wars have followed.
Hatred of Israel has actually created a new impetus for the US to act, in order to preserve its "regional hegemon" Israel. A regional hegemon is a country which attempts to exert control and influence over all other nearby states, like a mini-empire.
Client states can offer a great way for an empire to aggravate other empires, or contain their ambitions. If, for instance, one of Israel's neighbors negotiates with a rival like Russia or China to develop its oil fields, that neighbor might suddenly present some sort of terrorist threat that Israel will feel compelled to neutralize. Obviously the oil drilling poses no direct threat to Israel, but the mad dog approach acts to intimidate other nations in the region who might be thinking about an petroleum-based alliance with Russia or China.
Had the United States acted directly, a confrontation with the other empire--Russia or China--could arise, hardly an enviable situation considering the size and scope of two empires engaging each other. Instead Israeli bombs or missiles strike the "terrorist" targets, and if some engineers get killed, hey well that's Israel's fault, and the Arabs don't like them anyway, so, well , hey, we're sorry but maybe they shouldn't have been there in the first place.
The Chinese embassy got accidentally hit by a NATO airstrike in 1998, killing Chinese diplomat nationals. The incident just happened to occur after Chinese agents had reportedly stolen plans for the U.S. state of the art W-80 nuclear warhead. (The Wen Ho Lee witch hunt at Los Alamos had occurred at approximately the same time. Lee has been exonerated, settling for an undisclosed sum with the government as did Dr. Steven Hatfill after false allegations had been directed at him in the October 2001 anthrax cases recently attributed posthumously to Bruce Ivins.)
There's no way to directly connect one action with another, especially if the recourse comes through a third party like a client state. Initially, client states may not get much out of doing the dirty work for the empire benefactor. Still, the process seals the relationship between the empire and the client state. By acting on behalf of the empire, the client state can increase credibility in the region as its neighbors learn quite quickly that messing with the client state is the equivalent of messing with the empire.
Much of the empire's military power ends up being exercised indirectly by its client states. Other empires will act through their proxies--during the Cold War the Soviet Union sponsored numerous insurgencies, trying to destabilize the capitalist nations supported by the West.
Financial weakness
All empires have limits, even ours. While the US may have enjoyed a reign as the world's only superpower from about 1990 until recently, the rest of the world has caught up. The US once had a technological edge that's been lost to the newest empire: China. It has become dependent on foreign banks to supply its war machine, a problem which could make future borrowing subject to the political whims of the lenders, who may not be so eager to see their loans squandered on inflation-creating wars.
Domestic economic challenges often seal the fate of an empire. A great deal of future American wealth will disappear through government borrowing, and the implicit future tax burden. All extended wars in all previous empires have led to a debasement/devaluation of the empire's currency.
The empire's key source of strength is its economy, which are always undermined by ongoing conflicts and overspending. As the Roman treasury emptied, it devalued its currency, just as the dollar is crumbling. Nowadays, being based not on hard currency but fiat, the empire can always print up more money, but the consequences will eventually emerge in the form of inflation.
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