Okay kiddo, imagine two big countries that were fighting in a big war together that took a long time and a lot of lives. One of the countries, let's call it America, was powerful and won the war with the help of other countries. The other country, let's call it the Soviet Union, was also powerful but lost the war.
After the war, the two countries didn't always get along. They had different ideas about how to do things and sometimes argued with each other. The Soviet Union was especially worried that America might try to attack them because they were so powerful.
So, in a secret meeting in June 1945, some American military leaders came up with a plan called "Operation Unthinkable". The plan was to attack the Soviet Union with a huge army right after the war ended. The goal was to make the Soviet Union surrender and become friends with America again.
But the plan was very risky and could have started a new war. So, the leaders of America decided not to do it. They thought it was better to talk to the Soviet Union and try to work things out instead of fighting them again.
In the end, the two countries didn't go to war again, and even though they disagreed a lot, they managed to keep the peace. And that's how "Operation Unthinkable" was a plan that never happened!