These similarities recommend certain policy steps. Most important, Ukraine must succeed as a democracy, a market economy and a state. High on its reform list must be energy efficiency and diversification, as well as military and corruption reforms. Other exposed states in the region, like Moldova and Georgia, also need urgent bolstering.

Also, as during the 20th century, those states firmly on our side must be assured and protected. NATO has moved quickly already, but these efforts must be sustained through greater placement of military hardware in the front-line states, more training and integration of forces, and new efforts to reduce NATO countries’ dependence on Russian energy.

And, as before, the current regime must be isolated. The strategy of seeking to change Kremlin behavior through engagement, integration and rhetoric is over for now. No more membership in the Group of 8, accession to the Organization for Economic Cooperation and Development or missile defense talks. Instead there must be sanctions, including against those people and entities — propagandists, state-owned enterprises, Kremlin-tied bankers — that act as instruments of Mr. Putin’s coercive power. Conversely, individuals and companies not connected to the government must be supported, including those seeking to take assets out of Russia or emigrate.

Finally, as during World War II and the Cold War, the United States and our allies can cooperate with Mr. Putin when our vital interests overlap. But this engagement must be understood as strictly transactional, and not as a means to pull Russia back into accepting international norms and values. That’s how he will see this engagement. So should we.

At the same time, many important differences distinguish this new confrontation in Europe from the Cold War or interwar eras. Most help us. A few do not.

For one thing, unlike Communism or even fascism, Putinism has little appeal beyond Russia. Even inside Russia, brave civil society leaders still defy autocracy, war and nationalist fervor, and have managed to mobilize tens of thousands against Mr. Putin’s intervention, while a larger but quiet section of society will lament the advent of this new era.

I met these silent skeptics — in government, business and society — every day in my last job. Citizens rally round the flag during crises, and propaganda works. But Mr. Putin’s nationalism is fueled primarily by a crude, neo-Soviet anti-Americanism. To continue to spook Russians about American encirclement and internal meddling will be hard to sustain. They are too smart.