For once, the Serb pilots were caught in the act.
For nearly a year NATO has scoured the skies and filtered through unconfirmed reports, watching and waiting for a clear violation of a U.N.-imposed no-fly zone.
When it came, NATO and its warplanes didn't hesitate to use force and carry out the organization's first combat action since it came into existence 44 years ago.
Only two weeks ago NATO took a new, tougher approach to Bosnia's war with an ultimatum to Bosnian Serbs: Withdraw your heavy guns from around the besieged capital of Sarajevo or face air strikes. The guns were withdrawn.
NATO officials said Monday's air action was not the result of any new resolve by the 16-nation alliance or by the American, French, British, Dutch and Turkish jet fighters covering the no-fly zone.
This time the Serb pilots just did everything wrong.
"We did what we said we would do," said U.S. Adm. Jeremy M. Boorda, commander of NATO's southern European forces in Naples, Italy.
"Violations of the no-fly zone simply will not be tolerated."
Previous violations _ and there have been more than 1,000 _ were mainly by helicopters or fixed-wing planes on short non-combat missions transporting personnel or equipment. When challenged by NATO flyers, they landed or left Bosnian air space.
Although they saw frequent violations by helicopters and fixed-wing planes, the jet fighters had not fired a shot in anger.
"This was the first time it was a unit of armed planes in formation," said British Foreign Secretary Douglas Hurd of the Serb planes.
That, he said, marked a "difference in quality" that forced pilots in the two U.S. Air Force F-16s to launch their missiles against the Serb raiders over central Bosnia on Monday.
NATO Secretary-General Manfred Woerner told Germany's ZDF television that this was the first time in a year the alliance had clearly determined the violation of the no-fly zone by fighter planes.
"One will learn to take NATO seriously," Woerner said.
And this time, six Yugoslav-built Soko G-4 Super Galeb attack aircraft were caught bombing a government-held factory near the central Bosnian town of Novi Travnik.
NATO jets raced to intercept them. Twice, the U.S. pilots warned the Serbs to land, leave the no-fly zone or face attack.
Twice the warning was ignored.
So the U.S. planes fired their missiles and four Serb planes were shot down. The other two fled Bosnian air space.
Claims by the Bosnian government that Serb planes have bombed before have not been verified.
"This one, we were there, we saw it, so we know it was a real violation," said a NATO official.
Despite denials that the shooting does not mark a get-tougher policy by NATO, there have been signs the alliance and the United Nations have been loosing patience with the Bosnian Serbs.
The U.N. commander in Bosnia-Herzegovina, British Lt. Gen. Sir Michael Rose, decided last week he would no longer ask the warring parties for permission when he sends aid convoys though the battle-ravaged republic.
"It is a new resolve to get tough," said Jonathan Eyal, director of studies at the London-based Royal United Services Institute. "It's naive to suggest there was no political substructure to this."
A greater NATO involvement will have to contend with the reaction of the Serbs' traditional ally _ Russia.
Moscow strongly opposed the air strike ultimatum. But the Foreign Ministry backed NATO's decision to down violators of the no-fly zone.
"Whatever side has conducted a military flight over Bosnia in violation of (U.N.) resolutions regarding the no-fly zone, has to bear full responsibility for what has happened," it said in a statement.