system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How are ISIS making so many gains despite having seemingly poor training and fighting against official militaries?

I was always under the assumption that ISIS fighters were just civilians with poor training/equipment.
gpt
Many of them are ex-iraqi military, or insurgents who are battle hardened from fighting in Iraq since the US invaded in 2003. A bunch of them are basically just poorly trained civilians, but they've had some reasonably competent military leadership. 

But more importantly, they groups that they've been making progress against generally aren't any better trained/equipped/motivated. 

Much of the gains against in Iraq have been against Iraqi forces that, for various reasons, have barely bothered to fight. You don't need a great army to win against a force that runs away whenever you get close. 

On occasions where ISIS has met up with better organized forces, such as the kurdish peshmerga, they've had a much harder time making gains.