Former U.S. President Jimmy Carter says the Crimean annexation was "inevitable" because Russia considers the peninsula to be part of its country and so many Crimeans consider themselves Russian.
But he says President Vladimir Putin should not be permitted to go any further.
On the Late Show with David Letterman on Monday, Carter recounted when then-Soviet leader Leonid Brezhnev invaded Afghanistan in 1979.
Carter recalled the U.S. ambassador declared an embargo and withdrew from the Olympics. Then, Carter said, "We began to arm the freedom fighters."
He says that was "successful at stopping them there."
Carter says he believes no matter how President Barack Obama or European nations tried to head off the takeover, Putin would have gone ahead with it, but he should not be allowed to go further.