Let's Start with a Dirt Engineer and See What He's Made Of!

[Transmigration] [War of Resistance] [A book about fighting the Japanese]

Currently open worlds: [War of Resistance] [Great Ming] [Three Kingdoms game world]

The protagonist obtains the Time-Space Gate Master System, which lets him contact people in other worlds who possess subsidiary systems.

The War of Resistance world is the main storyline, and later on, the protagonist joins forces with the nation.

Protagonist: What? Killing Japanese soldiers boosts my constitution, too?!

Perfect! I'll settle old and new scores all at once!

Wipe them out completely. Every last one of them!