Now the debate has shifted to how Mr. Putin will do it: in one massive nationwide attack; a series of bites that dismantle the country, piece by piece; or a pythonlike squeeze. That last option is made all the easier with the news Sunday morning that Belarus is allowing Russian troops to remain indefinitely, where they can menace Kyiv, the Ukrainian capital. Mr. Putin might be betting that he can shatter Ukraine’s economy and oust its government without having to immediately roll in tanks.

Mr. Putin’s strategic choices over the next few weeks may make a huge difference in how the world reacts.

If he strikes to take the whole country in a single blow — the approach that senior American military and intelligence officials and many outside analysts now think is the most likely — it could provoke the largest, most violent battle for European territory since the Nazi surrender in 1945.

There is little question that the full package of sanctions and technology export cutoffs would be invoked almost immediately. International condemnation would follow, though Mr. Putin may be betting that it would not last long, and that the world would gradually get accustomed to a new, larger Russia reconstituting the sphere of influence that was once the hallmark of the old Soviet Union.