How can I keep an app open in the background?

As I was using an app on my iPhone, I accidentally hit the home button. I quickly reopened the app. But instead of it resuming where I left off, the app had to load all over again. (This was Clash of Clans and I was in a raid!) Is there a way to keep an app open in the background? Such a feature would also allow me to quickly check something and then return to the app that I was on.