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NATO's creation of a rapid-reaction "spearhead" force to protect Eastern Europe from Russian bullying reflects a cool-eyed calculation that Vladimir Putin and his generals won't risk head-to-head confrontation with the U.S. and its nuclear-capable Western European allies.

The new force will be small, with just a few thousand troops, but it's a powerful message from major powers that they're willing to follow through on NATO's eastward expansion with their own metal — and blood.

"Why would this be enough?" said Gen. Sir Adrian Bradshaw, NATO's deputy supreme European commander. "Well, precisely because in becoming embroiled in a conflict with capable combat forces from across the alliance, a potential aggressor recognizes that they are taking on the whole of NATO and all that implies."

"I don't think that anyone believes that Russia wants a strategic conflict with NATO," the British army general said. "Anybody would be insane to wish that."

The force was ordered into life Friday by President Barack Obama and other NATO leaders at a summit meeting in Wales to deter Putin and make NATO's most vulnerable members, such as Poland, Romania and the Baltic republics, feel safer from Russia's million-strong armed forces in light of Moscow's military involvement in Ukraine.

Russia seized the Crimean Peninsula from Ukraine earlier this year, and all signs indicate the Kremlin has been funneling troops, tanks and artillery to the pro-Moscow separatists who have been fighting government forces in eastern Ukraine over the past five months.

Ukraine is not a NATO member and not directly under its defense umbrella, but three other former Soviet republics have joined the alliance since the end of the Cold War, as well as the former Soviet satellite states of Poland, Hungary, the Czech Republic and Slovakia (formerly one country), Romania and Bulgaria.

Obama said the United States and other NATO counties are living up to their obligations under the 1949 treaty that gave birth to NATO.

"Article 5 enshrines our solemn duty to each other," he said. "An armed attack against one shall be considered an attack against them all. This is a binding treaty obligation. It is non-negotiable. And here in Wales, we've left absolutely no doubt. We will defend every ally."

Asked by The Associated Press what the U.S. contribution of troops and equipment would be, Obama didn't give specifics but said a "sizable portion" of the $1 billion in security aid for Eastern Europe he had announced in June will help finance the NATO Readiness Action Plan, of which the new force is a part.

British Prime Minister David Cameron said his nation will contribute a battle group — meaning about 1,000 soldiers.

Poland's Prime Minister Donald Tusk called the decisions taken by NATO leaders a "breakthrough" that will help make the Polish people more secure.

"This signal is very strong, and our eastern neighbor cannot ignore it," Tusk said, referring to Russia. "This signal means that NATO's security guarantees for Poland are no longer paper guarantees and are becoming practical guarantees. We should be happy about this. But we still have a lot of work to do to make the quantitative change satisfying."

Poland had wanted the permanent stationing of 8,000 or so NATO troops on its territory.

Initial reaction from Moscow was negative. Russian officials accused the U.S.-led alliance of using the crisis in Ukraine as a pretext to advance a longstanding NATO goal of moving its infrastructure closer to Russia's borders.

In a statement, the Russian Foreign Ministry warned that NATO's plans to conduct joint exercises in Ukraine later this month will "inevitably aggravate tensions, jeopardize the progress that has been made in the peace process in Ukraine and exacerbate the split in Ukrainian society."

One independent Washington-based analyst, dissenting from the NATO position, said he doubts the new force will be much of a deterrent to the Russian president's ambitions.

"It's already too little, too late," said Jorge Benitez, senior fellow for trans-Atlantic security at the Brent Scowcroft Center on International Security. "It continues to be an incremental and reactive response to what Putin is doing rather than trying to really make a strong statement."

Bradshaw noted that NATO's muscle consists of more than just the spearhead force: "It's many thousands of troops, and an enormous capability that follows on."

He said that details about the new force's composition and capabilities remain to be worked out, but that the unit should achieve "initial operating capability" in "a number of months, maybe a year plus."

Absent a crisis, most of the force would be garrisoned outside Eastern Europe, NATO officials say.

To help it get into action swiftly if needed, a command headquarters will be set up in Eastern Europe at a spot still to be determined, with equipment and supplies stockpiled in the region and planners brought in, NATO Secretary-General Anders Fogh Rasmussen said.

Along with the spearhead force, the Readiness Action Plan approved by Obama and leaders of the 27 other allied nations includes stepped-up intelligence sharing, an upgrade of defense plans, and more military exercises on short notice, Rasmussen said.

In addition, NATO air patrols over the Baltic and other air, land and naval measures already under way to reassure the Eastern Europeans will be extended indefinitely.

"This decision sends a clear message: NATO protects all allies, at all times," Rasmussen said. "And it sends a clear message to any potential aggressor: Should you even think of attacking one ally, you will be facing the whole alliance."

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Monika Scislowska in Warsaw and Julie Pace in Newport contributed.