But he says Russian President Vladimir Putin shouldn't be permitted to go any further.
On the "Late Show with David Letterman" 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 shouldn't be allowed to go further.