Hmm, Americans in Korea, Vietnam, Panama, Grenada, Suadi Arabia and now Afghanistan. Why yes sir, it looks like having a big military means you go where ever the hell you want and to the Abyss with anyone who gets in your way...
Very realistic of you, Sid.
But really, even little guys insist on "sneaking" forces through borders (much like Mexican day labourers). I'd take out their transports with privateers but they NEVER win, even against Triremes they get blown out the water (by arrows). What is the point having such a useless unit in the game? It's like a naval warrior that costs many shields to build.
Try fortifieing your entire border with mech. infantry in fortresses, like to see the AI not notice a border like that.
Or if they insist on pulling crap like that, launch your nukes and let God sort it out.