Joseph Stalin was very paranoid. He went to extreme lengths to avoid being assassinated or poisoned. These multiple layers of protection are detailed by Dnitri Volkogonov, in his book "Stalin: Triumph and Tragedy"

It was possible, as all things are. I know of only one truly active attempt to assassinate Stalin by the Nazi regime was "Operation Zeppelin", which failed rather spectacularly (the details escape me and I cannot find the text/book to cite).

It certainly is possible for nations to order assassins or spies to kill opposing heads of state. However, to do that, you need to have better intelligence than your target has counter-intelligence. Your assassins will have to overcome the enemy's own spies and assassins, and then find a way to get close enough to the target to kill them.

Heads of state generally have pretty high security, especially during wartime, so it's not exactly feasible to do.