Moldovan soldiers splash across a muddy Macedonian river. American infantrymen walk in tight patrol formation. A Hungarian squad watches a demonstration of mine detection.

This is European security today, NATO and non-NATO forces learning and working together.It's called Partnership for Peace, an American idea promulgated at a 1994 summit in Brussels. The purpose: bring European security firmly into the post-Cold War era to face the realities of the 21st century.

"The world has changed," said NATO Secretary General Javier Solana. "You have to be prepared to look at the challenges of the present and the future, not the challenges of the past."

At this sun-baked military base in central Macedonia, 13 NATO countries and 13 PfP members practiced peacekeeping skills this month in a weeklong exercise. It's exactly the kind of skills being used today in Bosnia, where 30 NATO and partner nations are keeping the peace.

"It's fun helping those guys," said 1st Lt. Wheeler Pulliam, 25, of Brookhaven, Miss., who had just conducted a demonstration in medical evacuation procedures before a group of former Warsaw Pact troops. "It's better to train them than to fight them."

It still shocks in many ways, helping former enemies get better at what they do or working together with forces that have always remained neutral.

In Bosnia, where NATO warplanes put an end to the fighting three years ago, NATO soldiers are working closely with old friends and former foes. The Russian brigade makes up part of the American sector in northeast Bosnia. The Nordic-Polish brigade brings together Poland, Sweden, Finland, Norway and Denmark - an old enemy, two old neutrals and two members of NATO.

This new cooperation became possible with the unraveling of the Soviet Union and the diminishing Russian threat.

The NATO task was once relatively simple - defend against a Warsaw Pact onslaught across the plains of Central Europe. Today, the threat is from regional conflicts like Bosnia or Kosovo, international terrorism, ethnic violence, the spread of weapons of mass destruction.

NATO's PfP program is trying to forge real partnerships with the 27 countries, including Russia, that have sought membership based on the belief that practical military and cooperation will lead to stability.

In exchange for a commitment to democracy, PfP members can get nearly everything NATO has to offer except automatic defense against outside attack. Many countries see PfP as a steppingstone to NATO membership.