President Donald Trump’s announcement on Monday about aid to Ukraine proves once again that he is nothing if not unpredictable.

If Trump has been consistent about one thing throughout his tumultuous, decade-long political career, it is support for Russian dictator Vladimir Putin and skepticism of Ukraine. In 2014, Trump praised Putin’s illegal seizure of Crimea — a prelude to Russia’s full-blown invasion of Ukraine — as “so smart.” Trump’s anti-Ukraine animus reached its nadir in February when he engaged in an Oval Office shouting match with President Volodymyr Zelensky. That led to a temporary pause on U.S. aid to Kyiv and could easily have signaled that the United States was abandoning Ukraine altogether.

And yet there Trump was in the Oval Office on Monday, meeting with NATO Secretary General Mark Rutte, and sounding a very different tone. The president announced that the United States would sell to European allies “top of the line weapons” that will then be provided to Ukraine, and he threatened to impose 100 percent tariffs on Russia and its trade partners if Putin does not reach a deal to end the war within 50 days.

Trump ultimatums are a moving target — as his repeated, oft-postponed threats to impose tariffs demonstrate — and there is still more the administration should do to aid Ukraine. But this is nevertheless a startling, and welcome, shift. It is good news for Ukraine and the cause of freedom — and bad news for Putin and his tyrannical allies in Pyongyang, Beijing and Tehran.

In part, this shift can be explained by Zelensky’s attempts to mend fences with Trump since their Oval Office showdown. Recall, for example, the now-famous picture of their conversation at Pope Francis’s funeral in April. Zelensky has shown that he can be a skillful diplomat as well as a fiery wartime leader.

But mainly this U.S. shift is Putin’s doing. Trump and Putin have had a half-dozen phone calls without making any progress on ending Russia’s war of aggression. As Trump complained on Monday, the phone calls are always “nice” but then “missiles are launched into Kyiv or some other city.” It has gradually dawned on Trump that the Russian strongman has been stringing him along and has no actual interest in ending the war.

Once again, Putin appears to have badly miscalculated, much in the same way that the Iranian mullahs miscalculated when they tried to drag out nuclear negotiations with the United States. Trump is showing that he is not afraid to punish countries when he concludes that they are not dealing in good faith.

Putin’s original, and most catastrophic, mistake, of course, was to imagine that he could capture all of Ukraine in a few weeks in 2022.

Instead, he mired his country in a war of attrition that has cost it around 1 million troops killed or wounded — more battlefield losses than Russia (or the Soviet Union) had suffered in all of its post-1945 wars combined. All of those losses, which include vast numbers of tanks, armored personnel carriers, aircraft, ships and other hardware, have not delivered the victory Putin seeks.

Since the heady initial days of the invasion, Russian forces have been advancing at rates slower than the glacial pace of offensives along the Western Front in World War I.

Rather than admit defeat, however, Putin has doubled down. He has increased the rate of missile and drone strikes on Ukrainian cities, and mobilized an estimated 600,000 troops in and around Ukraine. Putin still hopes that his armed forces, despite 31/2 years of battlefield futility, can defeat Ukraine, and he is willing to risk his relationship with Trump to give his military time to advance.

Yet, despite the numerical advantage that Russian forces possess, there is no indication they are at all close to a breakthrough. The front remains essentially frozen, because vast numbers of drones on both sides make it too perilous for troops to advance in large numbers. With Ukraine on schedule to produce 4 million drones this year, its forces should be able to hold off the Russian summer offensive.

The biggest risk to Ukraine in recent months has come from a potentially dangerous depletion of its air-defense ammunition, which has allowed massive Russian drone and missile attacks to inflict more damage on Ukrainian cities. Aerial bombardment is unlikely to win the war for Russia — the historical record of airstrikes, dating back to World War II, suggests they tend to strengthen, rather than undermine, the will to resist — but they could kill a lot of innocent people.

Now, however, Trump’s announcement that the United States will provide more air-defense ammunition, among other weapons, should blunt the threat of Russian air attacks. Critically, Trump indicated that he is willing to sell more Patriot systems to Ukraine.

This is an excellent start by Trump, but if he wants to bring the war to a conclusion, there is more he needs to do. The most important step, besides imposing the sanctions on Russia much faster, would be to provide Ukraine with frozen Russian funds that are being held in the West. Philip Zelikow, a veteran national security official who is now at the Hoover Institution, recently wrote that the amount of Russian funds under U.S. control is much higher than previously reported. Because so many of the Russian assets have matured from securities into dollar-denominated cash, Zelikow writes, the United States now controls around $50 billion in Russian funds. (Most of the rest of the $300 billion is held in Europe, though Japan, Canada and Australia also have significant holdings.)

If Zelikow’s estimate is accurate, that means the administration does not need to convince recalcitrant U.S. allies to seize a substantial amount of Russian funds; it can do so itself. Fifty billion dollars is an enormous sum that could provide Ukraine with a decisive advantage in the war. Ukraine has already done an impressive job of ramping up its domestic defense production, but Kyiv estimates that it could triple its output if it has the additional funds to do so. There would be poetic justice in using Russian assets to defeat the Russian invasion and bring Putin to the negotiating table.

Admittedly, it still seems far-fetched to imagine that Trump would seize the frozen Russian funds.

But until Monday, it also seemed improbable that Trump would provide any more aid to Ukraine. As Ayatollah Ali Khamenei and Vladimir Putin have now learned to their regret, it would be a mistake to take Trump for granted or to make assumptions about what he will or will not do.

Max Boot is a Washington Post columnist and a senior fellow at the Council on Foreign Relations.