Ex parte Milligan is a legal case that happened a long time ago during a war in the United States. In the case, a man named Milligan was arrested during the war because people thought he was helping the enemy. Milligan didn't get a trial with a jury of his peers (other regular people like him), which is something everyone is normally supposed to get when they're accused of a crime. Instead, some people in charge of the government decided that Milligan was guilty without a trial, and they wanted to punish him for helping the enemy.
However, Milligan and his lawyers argued that this was not fair. They said that the government was supposed to follow the law and let Milligan have a trial with a jury. They also said that taking away Milligan's rights like this was against the Constitution. The Constitution is like a big rule book that the government has to follow, and it says that everyone has the right to a fair trial.
Milligan's case went all the way up to the Supreme Court, which is a special court that decides who is right in cases that are really important. The Supreme Court agreed with Milligan and said that the government was not allowed to take away his rights without a fair trial. They said that even during a war, the government has to follow the law and make sure that everyone gets a fair trial if they're accused of a crime.
So, in a way, Ex parte Milligan was a really important case because it helped to protect people's rights during times of war. It made sure that everyone, even accused criminals, got a fair trial with a jury of their peers.