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The Pentagon Thursday unveiled sweeping plans to consolidate its forces in Europe, taking thousands of U.S. military and civilian personnel out of bases mostly in the United Kingdom and Portugal, in an effort that will save about $500 million each year.

Off-setting some of the troop reductions in the U.K., the U.S. has selected RAF Lakenheath in Britain to be the first permanent European base for the F-35 Joint Strike Fighter. The restructuring will take place over the next several years, and the first F-35 aircraft would arrive in the U.K. in 2020. They will replace F-15 fighter jets, which are leaving.

The changes involve mainly Army and Air Force personnel and facilities, and would result in the overall reduction of about 2,000 U.S. workers in the U.K. About 3,200 would come out of RAF Mildenhall while about 1,200 staff would be added at RAF Lakenheath with two squadrons of F-35 fighters.

The Pentagon has been systematically taking U.S. forces out of Europe in recent years, reflecting the ongoing decrease in the size of the Army and Marine Corps as well as the increased emphasis on the Pacific and a desire to shift additional troops into Eastern Europe and other regions where tensions with Russia have grown.

Many of the closures affect smaller bases that were remnants of the Cold War.

"We must seek greater efficiencies with respect to our presence in Europe and ensure we are focusing resources where they can have the greatest effect," said U.S. Air Force Gen. Philip Breedlove, the top NATO commander in Europe.

Derek Chollet, the assistant defense secretary for international security affairs, told Pentagon reporters Thursday that the consolidation will not affect U.S. military operations or America's ability to respond to its European partners. He said there are roughly 67,000 U.S. forces all across Europe, and since that total fluctuates as troops rotate in and out, that number will remain about the same.

Under the new restructuring, the U.S. would remove troops from about 15 bases, returning those installations to the host nations. While there would be additional base consolidation in Germany, the U.S. would end up with a few hundred more troops there after shifting some from the U.K.

John Conger, the acting assistant secretary of defense for energy, installations and environment, said it will cost about $1.4 billion to implement the closures and changes, with about one-third of that involving new construction and improvements.

As an example, Chollet noted that improvements are needed in some Eastern Europe nations where troops have been doing the more recent rotations.

U.S. officials also have finalized plans to cut about 500 military personnel from the Lajes military base in the Azores islands - a proposal that had drawn opposition from leaders in Portugal. Two years ago, then-Defense Secretary Leon Panetta met with officials in Lisbon about the plan, which was expected to save about $350 million over 10 years.

The U.S., at the time, agreed to reassess the move, but the Pentagon has now concluded that the plan to scale back personnel at the Lajes base should go forward. The plan reduces U.S. military, civilian and contract personnel there by two-thirds.

While permanent basing is declining, the military is ramping up its program to rotate forces in and out of Europe for training and exercises. Over the past year, the U.S. has sent a variety of troops, including special operations forces, to exercises and training in Eastern Europe, including Latvia, Lithuania and Estonia, as part of an effort to reassure allies in the region who are worried about Russia's increasing aggression.

The U.S. cuts can have a deep economic impact on the host nations, since they would likely trigger local job cuts, including workers who support base operations, maintenance and other services.  Conger said about 1,100 host nation jobs would be eliminated and another 1,500 would be affected, mainly because they would be moving to other locations in Europe.