As it was back in the middle ages, we may not have any other choice.
Sometimes the alternative to fighting is to become either a victim or a slave. I will be neither.
Paul has a habit of throwing in a final statement that can be interpreted a number of ways.
In this case, I'm going with a desire to be more successful in fighting our enemies than the Crusades were. Though it could easily be interpreted as an anti-war statement. I just don't think that's Paul's point of view however.