WASHINGTON (Reuters) - After more than a decade focused on combating Islamist militancy, Western military planners are once again contemplating potential war between major powers - and how to prevent one happening by accident.
Although the Cold War rivalry with Moscow has never been forgotten, current and former Western officials say Russia's annexation of Crimea has NATO powers tearing up strategic assumptions and grimly considering both conventional and nuclear fights.
As late as March, most NATO powers - with the exception of eastern members such as the Baltic States long worried by Moscow - had assumed Europe itself faced no imminent military threat.
It is still the case that few believe Russia would attack any NATO state, but, in order to deter, Western officials say they must consider and plan for the contingency.
The threat to U.S. allies in the Pacific from a stronger China has also focused military minds on how to contain the risks there, and ensure any localized conflict does not spill over into global war.
In a major foreign policy speech at the West Point military academy last month, President Barack Obama spoke mostly on counterterrorism and the Afghanistan withdrawal. But while he said the risk from other nations was now much lower than before the Berlin Wall fell, he made clear it still existed.
"Regional aggression that goes unchecked, whether in southern Ukraine or the South China Sea or anywhere else in the world, will ultimately impact our allies and could draw in our military," he told graduating cadets.
Tensions with Moscow and Beijing have increased faster than almost anyone in government in Washington expected. They are expected to dominate a meeting between Obama and Russian President Vladimir Putin in Normandy for the 70th anniversary of D-Day later this week.
Last weekend's annual Shangri-La Dialogue strategic conference in Singapore, meanwhile, showcased the growing gulf between Washington and Beijing on issues from regional maritime disputes to cyber security.
In recent weeks, current and former officials say, the Obama administration has been insistently reassuring allies and signaling foes where Washington's true red lines are.
Washington might not be prepared to act militarily in Ukraine but an attack on a NATO state such as one of the Baltics or a formal Asian ally like Japan, the Philippines or Australia would commit it irrevocably to war. Those treaty obligations are not new, but U.S. officials say it is important to make clear that they are taken extremely seriously.
They hope that will reduce the risk of an accidental war where a state takes action wrongly assuming other powers will not respond.
"It's not that the leadership in Russia or China is looking for a war - and the United States certainly isn't," says Kathleen Hicks, a U.S. undersecretary for defense until last July who now works for the Center for Strategic and International Studies in Washington.
"The real worry is miscalculation."