At some point he was a bodyguard for Rome, but the two became enemies. He delivered the fatal blow to Rome, and died himself sometime after. In later strips, it is shown that they are once again friends in the afterlife.
In a strip exclusive to the published version of the manga, it is suggested that Germania and Rome were allied as far back as the BC days of Rome, and that they presumably fell out in the AD era.
hope this helps you understand my Hetalia Grampa