When people or countries don't get along, sometimes they fight. This fighting can be very bad and can hurt a lot of people. The United States is a big country that wants to keep everyone safe, but sometimes other countries do things that the US doesn't like. When this happens, the US government can tell the other country that they are not happy and want them to stop doing bad things. If the other country doesn't listen or keeps doing bad things, the US government can decide to declare war on them.
Declaring war means that the US government is saying that they are officially starting a fight with the other country. It's like when you and your friend get into a fight and you say, "I'm not playing with you anymore!" but in a much bigger and more serious way. When the US government declares war, they are telling the other country that they are willing to do whatever it takes to stop them from doing bad things and to keep the people in the United States safe.
When the US declares war, it also means that the leaders of the United States are going to do everything they can to protect the country and its citizens. They will send soldiers to fight against the other country, and they will make sure the people in the United States are prepared for any attacks that might happen. It's kind of like when your mom or dad tells you to stay inside when it's dark, so you don't get hurt.
Overall, declaring war is a very serious decision that the US government doesn't take lightly. They only do it when they feel like they have to, and they do it to protect the people in the United States.