I don't think Obama is weak at all. He's stuck with his domestic policy commitments through thick and thin. He just doesn't care about foreign policy. As far as he's concerned, if Russia (or anybody else) overran Europe or the rest of the world, that would be fine with him, as long as he did not get blamed for it. He's the left-wing equivalent of Charles Lindbergh (meaning nations should prepare their own defenses or suffer the consequences) except he makes the usual noises about alliance commitments without meaning it.
IOW, he’s weak. Strong leaders act rather than talk.