I think the song is about the writer's inability to convey his feelings without saying "I love you" to his girlfriend
He is trying to explain his feelings for her but might as well be talking backwards because the "only thing that really matters" to his girlfriend is hearing I love you and that "is the one thing that (he) can't seem to do"
Also the imagery of war in the song is over him saying the words and "the line (that) is still engaged" is the metaphorical line of him saying I love you and she is "too many miles away" meaning she is set on him saying it no matter what other words he uses to express his affection
I think the song is about the writer's inability to convey his feelings without saying "I love you" to his girlfriend
He is trying to explain his feelings for her but might as well be talking backwards because the "only thing that really matters" to his girlfriend is hearing I love you and that "is the one thing that (he) can't seem to do"
Also the imagery of war in the song is over him saying the words and "the line (that) is still engaged" is the metaphorical line of him saying I love you and she is "too many miles away" meaning she is set on him saying it no matter what other words he uses to express his affection