Not sure if someone where read manga, but did just found this one:
http://www.batoto.net/comic/_/comics/gate-thus-the-jsdf-fought-there-r5353"In August of 20XX, a portal to a parallel world, known as the "Gate", suddenly appeared in Ginza, Tokyo. Monsters and troops poured out of the portal, turning the shopping district into a bloody inferno.
The Japan Ground-Self Defence Force immediately took action and pushed the fantasy creatures back to the "Gate". To facilitate negotiations and prepare for future fights, the JGSDF dispatched the Third Reconnaissance Team to the "Special Region" at the other side of the Gate."
Did like the focus of what would could happen in the begin if actual modern world did met a medieval fantasy one.
Do you guys know of another manga with same focus?