That huge mass of dark red is me, the Byzantine Republic. After Rome attacked one of my outposts, I invaded its continent with cavs, infantry, and artillery. Rome was the second-biggest world empire, with over 20 riflemen in its capital. HUGE army, but I destroyed them. I took
all of Rome, I endured waves of war weariness doing it, and when it was done I left the fascist Romans their island. Since I figured on Domination, I took some of the cities on that island for the peace, rushed Mech Infantry there so no one would see them and go "Oh, free city", and then invaded the Celts.
And now, having easily destroyed the Celts, the Romans -- these stupid, silly bastards -- invade the world-spanning empire with an industrial army when I'm about to finish the Manhattan project.

But they're JUSTIFIED. Why? Because their longbowmen...rout tanks. And their longbowmen destroy mech infantry. One longbowman, in one attack, took the city that has just been surprise-invaded. I had to drop off ten or so tanks to retake the island.
(I won Domination victory while my transports were transferring tanks from the occupied Celt zone to the Roman island, though -- a bunch of rushed temples caused borders all over to pop, and I got my 66. I still wiped out the Romans, though. This world is only big enough for one Roman empire, and that's the EASTERN one. )