The flock of Russian drones that entered Poland from Belarus on Tuesday night and Wednesday was a sharp reminder of how easily the war in Ukraine could set Europe ablaze.

Europe is a continent already on edge. Russia is militarizing, spending nearly 7 percent of its gross domestic product on the military and training schoolchildren in basic martial skills. There is widespread uncertainty about precisely what President Trump’s commitment to Ukraine, NATO and European security really means.

Polish, Ukrainian and many Western officials believe the Russian drone barrage was intentional, although Russia sent conflicting messages about that, and the top general of its ally Belarus said the drones had veered off course. But in any case, Russia got important intelligence for the future, testing and revealing NATO’s readiness and responses to any incursion.

Russia also sent a reminder to European leaders that their idea of sending troops to Ukraine as a security guarantee against Russian intervention would be very risky, given that any attack on them could also be put down by Moscow to errant munitions.

NATO said it was investigating whether the Russian drones were deliberately sent into Poland. But Mark Rutte, the secretary general, said Russia’s intentions were beside the point. “It is absolutely reckless,” he said. “It is absolutely dangerous.”

With Mr. Trump downgrading American financial and military support for Ukraine, President Vladimir V. Putin of Russia may be trying to test American responses, too, said Max Bergmann, director for Europe and Russia at the Center for Strategic and International Studies in Washington.

Presuming the drone attacks are deliberate, “Russia is pushing buttons, to see what the Americans will do,” he said. Like the Europeans, “Russia is trying to understand what the Trump presidency means and where is it headed,” he said. “And if you want to undermine NATO and Europe’s security architecture in a few years, you want to test it.”

For Piotr Krawczyk, former head of the Polish Foreign Intelligence Service, the drone incursion “was a deliberate act by Russia to escalate tensions and increase pressure on Washington,” carried out in the context of stalled U.S.-Russia negotiations on Ukraine.

“The ultimate target of these provocations is the United States,” he said. “Moscow seeks to pressure Washington by testing NATO, knowing that the U.S. will ultimately have to make the key decisions” about European security.

The Trump administration is conducting a review of American troops abroad, and the expectation is that thousands will be pulled out of Europe, either to be sent to Asia or returned home, where Defense Secretary Pete Hegseth has said “the real battles” are. That is why countries that are geographically close to Russia, like Poland and the Baltic nations, have dramatically increased their military spending, in part hoping to convince Mr. Trump to leave American troops there.

Poland is particularly engaged in a huge and expensive buildup of its own defenses. On Wednesday, it turned to its NATO partners for consultations under Article 4 of the NATO agreement, designed for a member that feels under military threat. Prime Minister Donald Tusk told his Parliament, “This situation brings us the closest we have been to open conflict since World War II.”

A line has been crossed, he said, and “it’s incomparably more dangerous than before.”

Mr. Trump, who has often criticized European countries for spending too little on defense, has spoken favorably about Poland and its commitment to military spending and deterrence. Poland, long mistrustful of Moscow, is spending some 4.7 percent of economic output on defense, among the highest in NATO.

Some 10,000 American troops are currently based in Poland. Last week, speaking alongside President Karol Nawrocki at the White House, Mr. Trump remarked that not only would U.S. troops remain in Poland, but “we’ll put more there, if they want.”

Yet he is expected to pull thousands more American troops from Germany and other countries.

Mr. Rutte spoke with urgency on Wednesday about the need to step up NATO’s game. “For NATO, we need to invest more in our defense, ramp up defense production so we have what we need to deter and defend, and we need to continue to support Ukraine, whose security is interlinked with our own,” he said. The episode underscores the need for NATO to beef up its defenses against modern drone warfare and learn the lessons from Ukraine.

Numerous NATO countries believe that once the fighting in Ukraine ends, Russia would be prepared to test NATO itself in the next five years, said Andrius Kubilius, the European Union defense commissioner and former Lithuanian prime minister. And they saw the influx of Russian drones into Poland as a provocation and a warning.

Kaja Kallas, the E.U. foreign policy chief and a former Estonian prime minister, said, “Russia’s war is escalating, not ending.”

Laurynas Kasciunas, a former defense minister in Lithuania, said, “Russia’s impunity in Ukraine allowed Moscow last night to go further and test NATO’s borders.” If there is no appropriate response, he said, “the attacks and provocations will only continue and escalate” to other NATO states.

Mr. Kasciunas called for NATO countries to deploy air defense systems right up to the eastern borders of Poland and Romania, to be used to help protect Ukrainian airspace, while also building up NATO’s own air defenses.

The incursion of the drones was also seen, according to two senior European officials, as a warning to European countries considering sending troops to Ukraine as some sort of security assurance in a peace deal, something Russia has consistently opposed.

The officials, speaking anonymously given the sensitivity of the topic, said that with Mr. Trump refusing to discuss American contributions to any such security in a phone call with European leaders last week, it was time to calm that discussion down and concentrate instead on supplying Ukraine now with the arms it needs.

For Russia, there may be another factor at work.

The drones came to Poland over Belarus. Mr. Trump has reached out recently to President Aleksandr Lukashenko about releasing political prisoners in return for American re-engagement and support for its sovereignty. Mr. Krawczyk, the former Polish intelligence official, said Russia was also seeking to undermine those efforts to preserve its domination of Belarus. “Russia aims to demonstrate that Belarus is merely a pawn in Moscow’s game,” he said.

Tomas Dapkus contributed reporting from Vilnius, Lithuania.