I am Christian but I do believe we have a duty to defend ourselves and our people from existential threats. The whole turn the other cheek line is meant for insults and petty offenses.
When your country is being invaded, and the invaders are raping your women and children, killing your brothers, you have a duty as a Christian to defend them, ideally non violently but there is no sin in taking arms and using violence when all peaceful options have failed.
There is a long history of great Christian generals and soldiers