Ukrainian President Petro Poroshenko, right, talks on Dec. 27 to Ukrainian military members released in a prisoner exchange with Russia-backed separatists. (Mikhail Palinchak/Agence France-Presse via Getty Images)

THE TRUMP administration hesitated for months over whether to provide Ukraine with defensive weapons, but when it finally acted this week, its timing was impeccable. Aggression by Russian-led forces in eastern Ukraine rose sharply in mid-December: Heavy artillery and rocket barrages belied the notion that Moscow's three-year-old intervention in the provinces of Donetsk and Luhansk had settled into a "frozen conflict." There was speculation that Vladi­mir Putin saw escalation in Ukraine as a way of rallying domestic support ahead of a March presidential election.

President Trump's decision to supply Ukrainian forces with Javelin antitank missiles and approve the commercial sale of sniper rifles, wouldn't stop a Russian offensive if there were one — but it could give Mr. Putin pause. It's a worthy application of the "peace through strength" principle of President Ronald Reagan that Mr. Trump says he admires. If there is ever to be peace in Ukraine — and an improvement in U.S.-Russian relations — Mr. Putin must first be made to understand that his aggressions, whether in Ukraine or in cyberspace, will be resisted and will incur tangible costs.

The quantities of U.S. weapons Ukraine is obtaining are modest. But officials call them “gap-fillers” because they remedy key weaknesses in the country’s defenses. On the eastern front lines, sniping is a daily peril for Ukrainian troops, who will be able to use the Model M107A1 sniper systems they are being sold to level the field. Similarly, the Javelins can take out Russian tanks in the event of a new offensive — and force the Kremlin to consider the potential cost in the lives of Russian soldiers it still denies are in Ukraine.

In authorizing the sales, Mr. Trump reverses the stance of President Barack Obama, who rejected proposals by his advisers and pressure from Congress to provide Kiev with lethal weapons. Mr. Obama reasoned that arms supplies would merely motivate further escalation by Russia. We will now see, at last, whether that was a sensible rationale or an excuse for inaction. So far the reaction from Moscow has been muted. The foreign ministry has issued predictable protests, but nothing has been heard from Mr. Putin. This week a large prisoner exchange between Ukraine and the Russian-backed separatists went forward without incident, and Secretary of State Rex Tillerson and Russian Foreign Minister Sergei Lavrov agreed in a phone call that talks on a peace settlement should continue.

Once the Potemkin show of the March 18 election is behind him, Mr. Putin may have reason to consider cutting his losses in Donetsk and Luhansk. To its credit, the Trump administration has made clear, most recently in an op-ed by Mr. Tillerson in the New York Times, that the normalization of U.S.-Russian relations cannot occur "absent a peaceful resolution of the Ukraine situation." Picking up on a statement by Mr. Putin, the Trump administration and European allies are offering to negotiate a plan for international peacekeepers who could deploy following a Russian withdrawal. If Mr. Putin really wants to reset relations with the West, he will have an opportunity. If not, it will be well that Ukraine will have more means to defend itself.