NATO would be likely to reinforce its troop presence in the Black Sea and the Baltics while fending off cyberattacks if Russia were to invade Ukraine, diplomats and former officials said.

But with the Western military alliance under no treaty obligation to defend Ukraine, which is not a NATO member, some of the toughest decisions could fall on the European Union.

They include how to hit Moscow with any new economic sanctions, the fallout from any shortage of Russian natural gas to Europe and taking in refugees fleeing war.