I have just read about the Lincoln assassination trial. Why were these civilians tried in a military court ?

I find it strange and unconstitutional that US civilians could be tried in a military court, without lawyers, not allowed to talk. They were all executed very quickly, obviously to cover something up. I have also read that official records of this trial are still classified and "may" be unclassified in another 100 years from now. What is going on?I have just read about the Lincoln assassination trial. Why were these civilians tried in a military court ?It was the quickest way to try them and execute them.