In the decades after the Soviet Union collapsed in 1991, Poland became perhaps the most pro-American country in Europe. It joined NATO in 1999, hosts some 10,000 American troops and has benefited hugely from U.S. political and military support.

Now, in just a dizzying few months, Poland has begun confronting a new era, with new anxieties. President Trump has threatened to abandon the longtime U.S. commitment to European security and is implementing tariffs that imperil the global economy. Russia’s president, Vladimir V. Putin, wants NATO troops out of Poland and has threatened further aggression beyond the war in Ukraine, Poland’s neighbor to the east.

Poland has responded forcefully. It is taking more of a leadership role in the European Union, increasing its already significant military spending and organizing a Swiss-style program of training ordinary citizens in civil defense. It is also cautioning countries in the rest of Europe that they, too, must pay more for their own security because the United States under Mr. Trump is no longer willing to foot so much of the bill.

An instructor leading a military techniques training session for civilians in Gdynia, Poland. The Polish prime minister, Donald Tusk, has called for military training for a month, with salary, for any citizen who wants it.Credit…Lukasz Glowala/Reuters

Security is perhaps the one issue that unites Poland ahead of a presidential election that begins in three weeks. More broadly, Poland’s location on NATO’s eastern edge makes it a crucial bulwark against Russian encroachment on Europe.

Prime Minister Donald Tusk of Poland declared in March that given “the profound change of American geopolitics,” Europe “would be safer if we had our own nuclear arsenal.” The statement shocked many because it implied that Poland and Europe could no longer rely on the American nuclear umbrella for protection.