Fortunataley there are multiple solutions for PEACE.
Assuming that the capital letters each stand for a digit (out of the range 0...9) there are exactly 5 solutions, and only one solution where P is not 0:
874+9546=10420, 812+6425=07237, 743+1639=02382, 892+5324=06216 and 892+6325=07217.
But there a set of solution!
Your first solution 874+9546=10420 is one of them! As with real countries!
812+6425=07237, 743+1639=02382, 892+5324=06216 and 892+6325=07217 - not correct!
Yes, this puzzle is unusual. A typical puzzle like this has only one solution. And this puzzle has eight solutions.
I do not know can we solve it without a computer.
Please show your solution by picture - I have not yet installed Prime 6 for myself.
My solution in attach.
From my book 2^5 Problems for STEM Education
"Rebus USA + USSR = PEACE is obsolete. First, there is no such country as the USSR anymore, and second, the solution of this puzzle is described in many entertaining books and even appears on the Internet—it is only necessary to enter (USA + USSR = PEACE) in any search engine.
More recently, and still unsolved, we can consider the problem† USA + IRAN = PEACE. This is unresolved in both the direct and figurative senses. In the figurative sense: taking into account the current state of US-Iranian relations."