When Ukraine’s military intelligence chief, Gen. Kyrylo Budanov, names specific dates for a Russian march on his country — late January or early February — it’s hard to believe Russian President Vladimir Putin will stick to Budanov’s schedule as if it were embossed on an RSVP card.

As I’ve argued earlier, such action would jeopardize a major part of Putin’s grand natural gas pipeline project, which is important to his legacy. And yet an all-out war in Ukraine is far from impossible.

Putin’s previous attacks on Ukraine followed two distinct scenarios. The Crimea annexation was a dazzlingly sudden grab. The Russian military involvement in Eastern Ukraine was, by contrast, reactive and perhaps even somewhat reluctant. It followed an attempt by armed groups of Russian nationalists with some initial backing from hawks in Putin’s own entourage to break the region away from Ukraine; after Ukrainian forces pushed back with surprising panache, Russian troops were sent in to save the secessionists.