Overextension frequently results from local failures of imperial management rather than simply "foreign policy" dynamics. The Spanish Habsburg's conflicts with England -- which scholars often cite as a key factor in Spanish overextension -- were, in part, a byproduct of a peripheral uprising in the Netherlands. Both Philip II and Philip III hoped that, by either conquering England or forcing it to capitulate to Spanish hegemonic coontrol, they could cut off England's strategic support for the Dutch (e.g., Allen 2000). Sustained rebellions represent, in fact, only an extreme case of these dynamics. As resistance to imperial bargains grows, empires will find it more difficult to garner and direct resources -- manpower, money, trade, and so forth -- from and toward peripheries. As their political capacity to manage peripheries diminishes they will, in turn, be more likely to suffer from overextension. Those who currently advocate American -- or American-backed Israeli -- military action against Syria and Iran embrace very similar reasoning to that of the Spanish: they argue that American problems in Iraq, and in the entire region, might be resolved if only the United States could neutralize those regimes that sponsor resistance to its objectives (e.g., Kristol 2006).
That's Daniel Nexon and Thomas Wright, "What's at Stake in the American Empire Debate?" available in your May 2007 issue of the American Political Science Review. More here.