[The writer is a history professor at Yale University.]

To govern, is to choose

This was the famous and ironic motto of French diplomats of the 17th and 18th centuries. As they saw it, in an anarchical international world, choosing priorities is tough. The rulers of national states, even those who appear strong and privileged, often find themselves forced to make hard choices. It may be better, then, to consider at an early stage how much one can take on. An incoming prince, or new head of a parliamentary government, might be advised not to undertake too many reforms on the home front, while also committing to hunting foreign demons abroad. A decision to withdraw from, or at least significantly reduce, an inherited policy could actually strengthen the leadership, by giving more space and energy to push through other ambitious plans. Pick your battles, and your terrain.

This general and cautious principle of not fighting on too many fronts at the same time has been much in my mind during the first year of the Obama administration. Does the president really believe he can achieve major reforms in healthcare, education, climate change, national finances and taxes, and win in Iraq and Afghanistan at the same time? What if the division of Washington’s energies leads to the sad result of being strong nowhere and weak – or compromised, or only half-achieving, or even failing – everywhere?

To this unpleasant general thought about the risks of striving everywhere and achieving nowhere should be added a more specific concern, relating to Barack Obama’s military “surge” in Afghanistan. There are good military, moral and strategic reasons why the new administration feels it has both to shore up the rickety Afghan government, and to put pressure on the Taliban and al-Qaeda, by increased operations in this region. It is clear that Mr Obama has consulted widely about this policy of escalation, and personally thought long and hard about such an unpleasant decision. But even that impressively intelligent man has not asked a further, most important question, which a person trained in military history and grand strategy would automatically be inclined to ask.

The question is this: are there military operations that Great Powers, even the greatest of the Great Powers at any given time, should not undertake? Are there campaigns that are just not worth fighting, because the terrain makes conquest impossible – or commitment of so many troops to handle such a treacherous operation would weaken obligations elsewhere? Does the number one power have to man every boundary, to be strong everywhere? Are there no limits?

History offers us a rich sample, of the more pragmatic of the Great Powers on certain occasions admitting they know their limits, whereas the intransigent and ideologically-driven kept declaring: “Never give up”. A “never give up” stance makes sense if a vicious enemy is bearing down on you with the intent of killing or imprisoning all your people. The Soviets had to fight a total war against Nazi Germany after the brutal invasion of June 1941; it was life and death. But the matter becomes more nuanced if it is about your commitment to a fight in foreign lands. Is the campaign really worth pursuing?

The Spain of Philip II and his successors sought for an extraordinary 80 years to suppress the protestant rebels of Holland and Zeeland, despite the fact that the waterways of the lower Scheldt made a land-based victory so dubious. The British abandoned their efforts to control America because the sheer distances and topography of the Hudson Valley and Appalachia made imperial control impossible by 1781-1783, at least at a price that London (then at war with France, Spain and the Netherlands) was willing to pay. After three successive Afghan wars, the British gave up any attempt to control that impossible terrain. The mighty Imperial Japanese Army’s efforts to conquer China between 1937 and 1945 foundered on the rocks of distance, climate, and logistics. China’s own many attempts over the centuries to punish the Vietnamese render a dismal record. Sometimes, it ain’t worth it. Sometimes, even, it cannot be done.

Smart, long-standing empires, such as that of the Romans, recognised their limits and rarely went beyond them. After losing three entire legions in the dense German forests, Augustus and his successors decided to establish a boundary along the western side of the Rhine. Similarly, the Danube became the barrier against the tribes of Dacia; the hairy barbarians could have the great Hungarian plain. Wales was unattractive and Scotland unprofitable, so the legions rarely went there. The north African coastal plains were rich in resources, but the Sahara to the south was an impassable barrier. Eastwards of Palestine was precarious, and the Persian Empire too big to pick a fight with – unless one took all one’s legions to the Euphrates and Oxus, like Alexander. The Romans were smarter than that. Thus were the limits of its influence set: by the Romans themselves. To stay strong overall, they were incredibly ruthless about where they would stay and fight, and where they would never again rush in. That, along with a few other things, helped the Roman Empire last for 500 years.

What does this sketchy historical record suggest for today’s number one power?..