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.