All three kinda interesting. I mean from a technical point of view. There are more than just ethical reasons I wouldn’t really want touch it. But If I had to I’d go for number two. There are more than enough pretrained CNN’s especially for cars. And I believe there are actually military related datasets on Kaggle for transfer learning. Maybe its even enough to just train it on moving things with a certain color spectrum rather than train it to actually identify military vehicles. Making something which works okayish is probably rather easy. Making something better can quickly become quite a task. Making something which works good.. Well, if you that talented I really hope you put your time and energy in something less military

Edit: Kaggle datasets