In a major policy reversal, the Pentagon has halted shipments of key munitions Ukraine needs to defend against Russia’s unprovoked aggression — a startling decision that runs counter to President Donald Trump’s “Peace Through Strength” agenda.

The immediate effect will be to weaken Ukraine’s defenses and derail Trump’s own efforts to achieve peace.

The potential long-term effect is equally damaging, undermining US reliability as a global power and a credible ally.

The suspended aid reportedly includes a variety of air defense missiles as well as artillery rockets and shells, some of which were already en route to Ukraine.

While the Defense Department cited concerns about American national security and depleted US stocks, many of these munitions were procured specifically for Kyiv under the Ukraine Security Assistance Initiative.

Others came directly from the Pentagon, pledged under so-called Presidential Drawdown Authority, after an official assessment certified that the transfer wouldn’t jeopardize US military readiness.

So this reversal implies some political thumbs on the Defense Department’s scale.

This follows a recent Pentagon decision to divert fuses for counter-drone rockets — also procured under USAI — from Ukraine to US forces in the Middle East.

This reduction in US munitions deliveries won’t lead Ukrainian lines to collapse.

It will, however, result in significantly more deaths of Ukrainian civilians and troops, more critical infrastructure destroyed, and more territory lost.

Vladimir Putin will view this Pentagon decision as a green light to ignore Trump’s frequent warnings to “stop.”

Ukraine’s defense of its skies will likely suffer the most.

Russia has recently ramped up its missile and drone barrages against Ukrainian cities and critical infrastructure, drawing on its increased production of these munitions as well as ballistic missiles from North Korea.

Kyiv, meanwhile, is already running low on key air-defense munitions.

It’s particularly short on interceptor missiles for the Patriot system, Ukraine’s main defense against ballistic threats.

Now, Washington is reportedly withholding dozens of those interceptors, which Ukraine’s European backers cannot soon replace.

Nor can they quickly replace the 250 GMLRS rockets reportedly blocked by the Pentagon, sharply reducing Ukraine’s capacity for long-range precision strikes on high-value military targets — a boon to Russia’s ongoing offensive.

Slashing Ukraine aid doesn’t just hurt Kyiv: It also damages Trump’s ability to achieve his stated goal of brokering a peace agreement.

Putin has made it clear he has no interest in peace unless it means Ukrainian capitulation.

For months, the Russians have dragged their feet on peace talks, insisting on maximalist demands while redoubling their assault on Ukraine.

As Trump himself has observed, Putin’s intransigence stems in part from the fact that Russia currently has the upper hand on the battlefield — and the autocrat seems to believe he can grind Ukraine down until it’s forced to accept his terms. 

By undermining Ukraine’s battlefield position and communicating that American support is waning, the Pentagon’s decision will only harden Putin’s determination and undercut Trump’s leverage.

Indeed, the Kremlin quickly welcomed the aid suspension, saying brings Russia “closer” to ending the war.

Long-term, this Pentagon decision will do lasting damage to America’s reputation as a credible ally, reversing the gains Trump achieved with his strong support for Israel and attack on Iran’s nuclear installations.

The president’s recent decisions to send an additional THAAD missile defense battery to Israel and surge Aegis destroyers to the Eastern Mediterranean undoubtedly risked US military readiness in other theaters — but Trump made the leap, showing strength to the Iranian mullahs and support for our Israeli ally.

By contrast, cutting assistance to Ukraine will shake confidence in the United States across the globe.

Having come so far to support Ukraine’s fight for freedom, pulling the rug out now is unconscionable.

Trump will have to own the strategic and political disaster the Pentagon is courting in Ukraine — much as President Joe Biden had to own the fallout from the botched US withdrawal from Afghanistan.

Rather than slashing Ukraine aid, Trump should seek to help Ukrainian forces exhaust the Russian military’s offensive potential as soon as possible.

In concert, he should work with America’s Western allies to ramp up economic pressure on Russia.

Trump is no isolationist.

He proved that when he bombed Fordow.

And to be sure, his Defense Department is right to put American warfighters first.

But given what’s at stake, the president would do well to reconsider whether the United States gains more from releasing these weapons to Ukraine than holding them in reserve for a rainy day.

Rear Adm. (Ret.) Mark Montgomery co-leads the Air and Missile Defense Program at the Foundation for Defense of Democracies, where John Hardie is deputy director of the Russia Program.