There are two important lessons of history for an imperial strategist who wants to avoid the trap of overreach.
The first is not to risk engagement in a new theater while an old crisis remains unresolved. Philip II of Spain sent the Armada to her doom while the rebellion in the Low Countries was still in full swing. Napoleon turned against Russia without ejecting the British from the Iberian Peninsula first. Hitler left the job in the Mediterranean and North Africa unfinished when he launched the Barbarossa campaign.
The second is not to risk power projection far from home without an assurance of support from reliable allies in the theater. The Athenians’ Sicilian expedition ended disastrously because their local allies were weak and prone to change sides. Hannibal marched across the Alps and initially scored three great victories, but in 15 years of operations in Italy he was unable to put together an effective anti-Roman coalition. More recently, the U.S. failure to win hearts and minds in Vietnam was a key cause of defeat.
Defense Secretary Ashton B. Carter is a major advocate of American primacy, which is a newly popular term for the old imperialist notions of “benevolent global hegemony” (William Kristol and Robert Kagan, 1996) or “full-spectrum dominance” (George W. Bush’s 2002 National Security Strategy). The concept...