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[The Valkyrie]

I was thinking something like Hadrian's Wall.  A tall, fortified wall running along the entire border, with garrisoned fortresses every couple of kilometers, manned by volunteer groups of heavily armed militias.

Back this up with drones flying support and attack helicopters.  Possibly even with gun emplacements, minefields, and tanks.

If you're familiar with it, perhaps even something like Mega-City One's West Wall.

Okay. My idea was shit.

I overthink things...