Okay kiddo, you know how sometimes when you play a game with your friends, one of your friends might try to sneak up and look at your cards without you noticing? Well, imagine if there were two countries playing a game, except it wasn't a fun game like Go Fish or Uno, it was a serious game called war.
One of the countries, Russia, wasn't very happy with the other country, Ukraine, and wanted to do something sneaky to try and win the game. So they sent some of their spies, who are kind of like secret agents, to Ukraine to try and help them win.
These spies tried to do things like gather information about what Ukraine's army was doing, and maybe even try to sabotage them too. This is called espionage and it's against the rules of the game, just like looking at someone's cards without them knowing is against the rules of your game.
But Ukraine found out what Russia was doing and they didn't like it. They told Russia to stop, but Russia didn't want to. So Ukraine asked for help from some other countries who were playing the same game, called NATO.
And just like how your mom or dad might come over and tell your friend not to cheat in your game, NATO helped Ukraine push back against the Russian spies and tried to make Russia play fair.
Even though it's not a fun game like the ones you play with your friends, war is still a very serious game and it's important that everyone follows the rules so no one gets hurt.