So yeah, a small rundown.

1. “Gate - Thus the JSDF Fought There!“ written by Takumi Yanai, it started in 2006.
2. “The Salvation War” written by Stuart Slade, it started in 2008.

Them one big similarity that connects both those titles is the “XX/XIX century military curb-stomping ‘medieval’ style invaders, then counter-invading” motive. Further on everything is different.
In Gate the invaders are a cliché fantasy empire with all the usual mix of creations while being quite low-magic. In TSW the aggressors are Demons straight from hell.
TSW is also written like a war commentary instead of a typical novel (think Cornelius Ryan “The Longest Day” and “One Bridge Too Far”).

Really hoping for more of such titles in the future. Of course there is also David Weber and Linda Evans with their "Multiverse" series but unfortunately it's not so good.

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David Weber and John Ringo's Empire of Man series has a high tech group cast into a medieval world which they journey across (books 1 to 3). Really good series which I greatly enjoyed.