Alliances

Is it possible to force an ally to wage war against any one of your enemies?

nope.

No diplomacy here, move along, move along.

Has anyone ever been given the option to help an ally (by moving in troops) when they are invaded? This hasn't happened to me yet, just curious.

When your ally is besieged, you can move troops in and rout the seaging army. Other than that, I have "helped" allies only by accident...