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Czarek Sokolowski, Associated Press
U.S. and European troops stand at attention during the opening ceremony of the major international Anakonda-14 defense exercise at the National Defense Academy in Warsaw-Rembertow district, Wednesday, Sept. 24, 2014. The 10-day exercise of the army, navy and airforce from Poland, U.S. Canada and six other countries from northern and eastern Europe is to test their readiness to jointly react to security threats at a time of armed conflict in Ukraine, that also involves Russia.

WARSAW, Poland — Poland launched a major military exercise with international allies on Wednesday to test joint response to security threats at a time of armed conflict in neighboring Ukraine.

Defense Minister Tomasz Siemoniak said the original plan for the biannual Anakonda exercise was to involve only Polish command officers.

But because of the Ukraine-Russia crisis and general fears that the conflict could spread to bordering countries, he decided it should have wider participation and should test collective defense readiness in accordance with NATO guidelines.

The Anakonda-14 exercise involves 12,500 troops, including about 750 from the U.S., Canada, Britain, the Netherlands, the Czech Republic and Hungary, as well as Lithuania and Estonia, which border Russia.

"Poland's biggest military exercise, Anakonda, has acquired special significance due to the events in Ukraine," Siemoniak said during the opening ceremony at the National Defense Academy.

"This is the first exercise on this scale since the conflict began" in mid-April, Siemoniak said.

Many Poles and people in Baltic countries are nervous that Russian aggression could spread beyond Ukraine. Despite a Sept. 5 cease-fire agreement between Ukraine and pro-Russia rebels, shelling continues in areas of Donetsk, an eastern Ukrainian city.

The scenario for the 10-day exercise calls for the protection of borders and will be held at four test ranges in northern Poland and on the Baltic Sea.

The exercise will use some 120 armored vehicles, more than 50 anti-aircraft units, and 15 missile launchers. A submarine and a missile frigate will be among the 17 warships training in the Baltic, while the air force will use 25 aircraft, including F-126 fighters, helicopters and Hercules transport planes.