Former Defense Secretary Robert Gates says Russian President Vladimir Putin wants to recreate a Soviet-style sphere of Russian influence in Eastern Europe -- an ambition the United States and its allies must keep in check.

Gates told "Fox News Sunday" that Russia's current military occupation of Ukraine's Crimea region is an example of Putin's long-term strategy for Russia to have "economic, political and security relationships with these countries that make them all lean toward or do the bidding of Moscow."

Putin "may retreat tactically from time to time, but this is part of a longer-term effort to stop the expansion of NATO, but more importantly, bring the states of the former Soviet Union back under the influence of Moscow," he said.

Gates, who served as defense secretary during the Obama and George W. Bush administrations, said Putin's push into Ukraine won't stop until the country has a pro-Russian government.

When asked if he thought it was inevitable that Russia would annex Crimea, Gates said "I do."

Gates said a key in thwarting Putin's ambitions in Eastern Europe is to help the region become less dependent on Russian energy, including the construction of a new pipeline to bring natural gas into Europe.

"What we need to do is show Russia that there are long-term consequences to this aggressive behavior on their part," he said. "Our tactical options are pretty limited."

While Gates stopped short of calling for U.S. troops to become directly involved in the Russia-Ukraine conflict, he said the U.S. should provide military assistance to Eastern Europe, particularly Poland and the Baltic nations of Latvia, Lithuania and Estonia.

But the former defense secretary said Putin likely doesn't want to re-create a full-fledged recreation of the Soviet Union "because he doesn't want to have responsibility for economic basket cases, like Ukraine is at the present time."

"What he wants is for those governments to look to Moscow and basically subject to whatever Moscow wants, without having responsibility for them," he said. "Whether something like that can happen in the near-to-medium term, I don't know."