Lest Putin seize upon these developments as an opportunity to intensify his aggression against Ukraine (which is highly likely given his recent behavior) and feel even more emboldened not to negotiate a constructive outcome, this would be an ideal moment for President Trump to move boldly to disabuse him of such notions.

The president has a fondness for associating himself with “doctrines” of highly regarded predecessors.

President Trump has drawn on and endorsed the Monroe Doctrine — rebranded as the “Donroe Doctrine” — to project American power and influence in our hemisphere. The Donroe Doctrine demonstrated that invoking a historic foreign policy framework — and updating it to contemporary circumstances — can be a force multiplier for American leadership. It rallied domestic support, signaled resolve to adversaries, and provided a clear picture of American intentions in the Western Hemisphere. Europe now needs the same clarity.

That clarity has a name: the Truman Doctrine. Current circumstances in Europe provide the president with not just an opportunity but an urgent need to associate himself with this historic American foreign policy framework as well — one purpose-built for exactly the kind of Russian aggression now unfolding.

Doing so would be a vivid and compelling demonstration that the U.S. is not being diverted by the Iran War from countering Russia’s war of aggression in Ukraine. It would, appropriately, underscore that the U.S. — even while engaged in the Iran War — continues to value NATO and recognizes Russia as a longer-term strategic threat to American and global interests.

Russia continues to attempt to undermine NATO, destabilize Europe, threaten its smaller neighbors, and position itself to dominate the Arctic. It seeks to weaken international controls on nuclear weapons, use cyber to threaten our infrastructure, undermine and manipulate American and other Western elections, and engage in massive information warfare.

Truman’s powerful message to what was then the Soviet Union was contained in a speech delivered before a Joint Session of Congress in March 1947. At that time, the Soviets were occupying several countries in East and Central Europe, and half of Germany. The KGB Soviet security services were undermining governments in others. Yet because the Soviet Union was a major ally in WWII, many Americans — including high-ranking officials who remained from the Roosevelt Administration — argued that the U.S. could “work with” Stalin to convince him to allow democracy in the countries he occupied and TO withdraw his troops. (There are advocates of similar sentiments in Washington today — suggesting that a fair peace in Ukraine and other constructive steps can be produced by “working with” Putin.)

Truman was convinced that Stalin had no such intentions, that his military was poised to take additional territory, and that the Soviet security services would undermine governments of additional countries such as Greece and Turkey.

Truman insisted that the softer America was in dealing with the Soviets, the bolder the Kremlin would be. It was in America’s security interests, he believed, to resist Soviet efforts to sustain and increase their military presence and political influence.

“I believe,” Truman declared before Congress, “that it must be the policy of the United States to support free peoples who are resisting attempted subjugation by armed minorities or by outside pressures.” He received a standing ovation on both sides of the aisle and bipartisan Congressional support for substantial resources to back him up.

This is a compelling moment to revive the Truman Doctrine. While the world has changed enormously since 1947, and Europe now is assuming most of the financial burden for its own security as well as for supporting Ukraine, the Kremlin remains defiant. It consistently undermines American and European efforts to achieve a just peace in Ukraine and represents a longer-term threat to other countries in the region. It still appears to have an appetite for sustained aggression. And it constantly throws out reminders that it remains a nuclear power.

Moreover, there is considerable evidence, and there are numerous disturbing reports by our closest allies, that Russia has provided Iran with intelligence to target U.S. weapons and bases that house large numbers of American troops. In one particularly egregious case, Iranian missiles and drones attacked and damaged a vitally strategic American AWACS plane at a Saudi base.

Clearly, the efforts made by the U.S. so far have had little deterrent effect and have not moved Putin toward constructive policies in Ukraine or the region. And Washington officials, in public at least, seem nonchalant about Russian actions that endanger American lives in the Gulf. Perhaps this will be the last straw demonstrating the need for bold U.S. leadership in dealing with Russia of the kind exhibited by Truman decades ago.

Even the most compelling “doctrine” will not have a decisive impact without robust armed support. But it can provide a strong rationale and moral foundation for that support. The muscle behind the Monroe Doctrine came not from America’s Navy — we barely had one at the time. It came from the British Royal Navy; Britain shared America’s interest in preventing Spanish recolonization of Latin America and extinguishing their new democracies. The Donroe Doctrine, therefore, derived its force not from American doctrine alone, but from support of A partner who shared the same interests. Allies were deemed important then, and Monroe knew it. Trump should know it too.

Reviving the Truman Doctrine would be a strong signal of American resolve in Ukraine and elsewhere — the more so if it were backed with substantially increased military support for Ukraine to further degrade the oil infrastructure that finances Russia’s war as well as to enhance sales of longer-range weapons for Ukraine. Just as the Donroe Doctrine secured America’s backyard, a revived Truman Doctrine — call it what you will — must now secure Europe and the broader democratic world.

President Trump has already endorsed one historic doctrine in our hemisphere. The Iran War and Putin’s opportunism demand that he do the same for Europe. The architecture is familiar. The precedent is his own. The moment is now.

Strong leadership based on strong principles could mobilize greater public and political support in the U.S. and elsewhere for confronting Russian aggression in Ukraine and security threats in other parts of the world. The Truman Doctrine made a consequential mark in history by standing up to Moscow’s threat to democratic nations decades ago. Giving it new life now would demonstrate a willingness to draw on its principles and resolve to confront a challenge likely to face the U.S. and the world long after the Iran war ends.

The opinions expressed in Fortune.com commentary pieces are solely the views of their authors and do not necessarily reflect the opinions and beliefs of Fortune.