Arresting people occasionally puts innocents in jail.
It's impossible to completely avoid civilian casualties in war unless you conduct absolutely no military operations whatsoever. The subtext of this is, of course, that the US should have avoided this, but how? Never go to war? That's obviously impractical.
Okay, so how about only going to war when you have a really good reason? If that's your plan, and you do approve of war as long as there is a really good reason, then (since some civilian casualties are inevitable) you've just said that you're okay with civilian casualties as long as the war is for a really good reason. Needless to say, you never see anti-war people saying this.
Being more careful in war? Well, you can be more careful, but nobody's perfect; there will always be *some* civilian casualties. So you're not really objecting to civilian casualties; you just think there are too many, but fewer but still some is okay. I've never seen anti-war people saying that either.
So what exactly should be done, other than never going to war, ever?