To safely invade it, you need strong enough armor, which means tanks. But if you're feeling brave, there do exist other strategies.
The minimal tech strategy involves bringing a CONVOY full of ubers in BRAINER outfits, with maxed out TU, Strength and Throwing, and/or catgirls with CAMO PAINT /CAT. Why these two outfits? Because with their anti-camo/camo they outrange the night vision of Ninja Warriors. NIGHT OPS is also possible with Nightingale 2+, and preferred so you aren't gimped with 1h weapons. So you attack the base at night, you take out the light balls (with DB Shotgun) on your spawn, and then attack Ninja Warriors from 14 (cats from 10) tiles away. Throwing Hammers, Throwing Axes, Javelins are all effective weapons. You rush next to the bunkers, kneel so that that turrets can't shoot you. Next turn you open up the bunker walls and defeat the rest of the Ninja Warriors inside. Then you use these bunkers for camping and take out the rest of the base.