@revelette1,
Assassin comes from the Arab Hashisheen, and assassination was very successful in the Arab world. If you are to take out the leader of an army then the army stalls, there is a struggle for power, and the new leaded may not be as in lined tonwar as the old one.
It was less successful against the Christian crusaders, they had an established hierarchy, when a leader died they knew who would take their place, there was no power struggle.
That's probably the case in Eastern Europe right now. It depends on how committed Putin's inner circle are but if Putin were to die suddenly it could well be a game changer, with the ruling classes choosing to cut their losses pull out and pin everything on the dear departed leader.
If Biden were taken out, it wouldn't change a thing. Kamala Harris would become president and US policy towards Europe, NATO, Russia and Ukraine would remain the same.