HE INVADED Crimea a year ago and then formally annexed it in a brazen, illegal act of aggression not seen in Europe since the Second World War. He sent thousands of Russian soldiers across the border to tilt the balance of Ukraine’s civil war in favor of pro-Moscow separatists and then refused to own up to it in a Big Lie reminiscent of Stalin’s days.

He gave rebels the sophisticated weapons that shot the Malaysian airliner out of the sky in July and have pulverized Ukrainian villages and towns.

He committed solemnly to a cease-fire agreement in September and then proceeded to violate every bit of it. He has just convinced German Chancellor Angela Merkel and French President Francois Hollande to agree to yet another cease-fire, one that is already in danger of unraveling.

After all he has done overtly and covertly to subvert Ukraine’s independence, why would Merkel and Hollande believe a word Vladimir Putin says?

Putin’s ambition is clear. He wants to dominate all the former Soviet states to Russia’s south and west in order to create a buffer zone that will insulate his authoritarian regime from the infection that might eventually destroy it — independent, free market, democratic governments.

That is why he has intimidated client states like Armenia and Belarus from pursuing closer association with the European Union. It is why he has fueled territorial divisions in the Transnistria region of Moldova and why he invaded Georgia in 2008 in support of the bogus sovereignty claims of Abkhazia and South Ossetia.

Similarly, he now wants to divide and destabilize Ukraine on a permanent basis to create yet another frozen conflict so that a weakened and discredited government in Kiev cannot succeed outside Russia’s orbit.

In response, President Obama and Merkel made a wise decision: They would not fight Russia for the future of Ukraine. But they’ve been two steps behind Putin throughout the crisis. They imposed a series of economic sanctions in 2014 that weren’t strong enough to deflect the Russian leader.

If it now turns out that Putin and separatist forces violate the latest cease-fire agreement, a much stronger western response will be needed. Obama doesn’t want to break completely with a Russian government critical to the Iran nuclear negotiations. But if we do nothing, Putin will have effectively drawn new dividing lines in Europe of the type we thought had disappeared forever with the end of the Cold War.

Merkel and Obama can’t let that happen. Unless we build a solid brick wall of opposition to his latest power grab, he will continue to dominate Ukraine and might then seek to subvert NATO allies Estonia and Latvia on his western border.

That’s why Obama and Merkel should consider more aggressive financial sanctions, which, combined with falling oil prices, will deepen Russia’s economic crisis. They should increase dramatically international economic support for Kiev. Obama should also up the ante by delivering powerful defensive weapons to the embattled Ukrainian government. That will drive up the costs to Putin, who respects power above all else. It will also give the Ukrainian government a fighting chance to defend its cities and towns under assault by the rebel army.

The United States is locked into another generational struggle with Russia for power in Europe. We ultimately triumphed in the Cold War because stalwart American presidents from Truman to Kennedy to Reagan did not flinch in the face of authoritarian aggression. If we are patient, strong, and unyielding, we will win this struggle too, because of our stronger, democratic values, our more successful economy, and the strength of our NATO allies against the sour, cynical, and soulless Putin regime.