Between the election and the inauguration, when Donald Trump was still praising Vladimir Putin as "very smart," I wrote a column in the Washington Examiner predicting that their relationship would sour. How did I know? Not through any Nostradamus-like prophetic powers. It's simply that every U.S. administration follows the same trajectory vis a vis Russia's strongman. George W. Bush began by calling Putin "very straightforward," and ended by arming Georgian troops to defend their country against him. Hillary Clinton began by offering him the famous "reset" button, and ended by imposing economic sanctions.

With President Trump, the falling out came more quickly. During the election campaign, the Donald was so warm in his rhetoric about Putin that many of his critics believed he was elected with Russian help. He described Putin as "doing a great job" and hoped he would be "my new best friend." As recently as last month, the president came close to saying that America was morally no better than Russia when it came to having people assassinated. Now, following the U.S. missile strike in Syria, we are back to Cold-War-style hostility on both sides.

American presidents come and go, but Putin stays, inscrutable behind his Dobby the Elf visage. He has taken the same line with each new leader of the free world: to pocket their concessions and carry on as he pleases, gobbling up neighboring real estate, arming tinpot tyrants, violating treaties. There is a reason that all attempts to draw Russia into an alliance have failed; and that reason is not a lack of will on the Western side.

Like most businessmen, Donald Trump knows how to drop a bad idea quickly. If detente doesn't work, containment becomes more important. Meeting the secretary-general of NATO, Jens Stoltenberg, on Wednesday, the president announced a massive volte-face. "I said it [NATO] was obsolete. It's no longer obsolete."

Which is a rational response to new conditions, I'd say. A decade ago, NATO did indeed seem obsolete. It had, after all, done its job: Western Europe was no longer threatened by Soviet T-72s. NATO existed, not for any obvious purpose, but through bureaucratic inertia: Few international organizations offer to disband. NATO apparatchiks justified their salaries by directing military interventions in Kosovo and Afghanistan. But, as NATO itself frankly admitted in 1999, "large-scale conventional aggression against the alliance is highly unlikely."

Since then, Ukraine has seen some pretty large-scale conventional aggression. In fact, Europe hasn't known such intense fighting since 1945. Can you be certain that Putin would treat, say, Latvia as being in a wholly different category simply because it's in NATO?

NATO members are supposed to treat an attack on one member as an attack on all. But what, these days, constitutes an attack? It's hard to imagine the Russian ambassador to Riga solemnly presenting a declaration of war. But is it so hard to imagine ethnic Russians in that country forming "self-protection units"? To imagine those units taking control of certain parts of the country and declaring them autonomous? To imagine Russian "volunteers," who just happen to be off-duty soldiers, crossing the border to support them? To imagine, in short, a Ukraine-style invasion by stages?

Now ask the question: At which of those stages would NATO consider that it had been attacked?

I'm not sure the answer is clear, either to Washington or to Moscow. And lack of clarity in these situations can be catastrophic. Uncertainty over whether Britain would stand by its guarantee to Belgium played a part in causing World War I: "You have your information, we have ours," the German ambassador to London told his French counterpart shortly before the fighting began. A similar doubt existed over the Western commitment to Poland in 1939.

Britain is hurrying to dispel the ambiguity. It has deployed soldiers to defend Estonia. It pressed for sanctions against Russia at last week's G-7 meeting – though, not for the first time, its Continental allies wanted a softer line. None of that counts for much, though, without unequivocal American support.

Donald Trump's newfound support for NATO delights the governments of Eastern Europe — and may encourage reformist Russians, who have been protesting in unprecedented numbers against a regime whose contempt for international norms has led to isolation and corruption.

The challenge, now, is for the other NATO allies, of whom only two meet the minimum defense spending of 2 percent of gross domestic product. Britain and the United States have proper defense budgets because they are committed to Europe's security. Is Europe?

Dan Hannan is a British Conservative MEP.