ok... when you have like 2 allies (like its a 3 vs 3 game) and you can change sides heres what you do.

1) you make a military slowly but at a steady pace.
2) you garrison them in cities, forts, and towers
3) You keep a small amout on the border of you ally so he doesn't try to invade you cause your weak.
4) Get a war started with your enemies and make sure your allies get attacked
5) As soon as you (your allies) start winning the war tell them that you will help defend the "homelands" and withdraw your small army (while the rest are still garrisioned) and rebuild it.
6) Once you see that the war is going well for your side switch sides and ungarrison your units and attack your ex ally.

Now he's got a real problem. Either he can continue he attack on the "enemy" or he can withdraw and attack you either way he's gonna have an army on his butt.

7) Now tell your enemy that you saved his butt and that you 2 should ally up and defeat your old allies. This will set you up for later in the game. If they don't ally up with you they ahve to worry aobut the army driving into thier country and everyone will be thrown off by your attack and will ahve no power to stop you.
8) After all this you should either have a new group of allies who are weaker than you and you should be conquering the old allies (you have to send some of your army to help them so that 1 the enemy has a weaker army and 2 to get him to ally with you) , or you should be conquering the world with all of your army.

I've tried this a couple of times and it works ok but I just want to see what ya'll think.

Um that would be considered backstabbing, and the game will end up you vs. every player on the map in a hurry.

Here's a few points:

1) Making sure you allies get attacked will make them wonder why you're doing such and will grow suspicious of you.

2) If you and your ally have the upper hand and are winning, all of the enemies' resources will be spent defending their base, thus your ally won't believe that you need to protect your area.

3) Finally, here's a dose of common sense, if your enemies see that even your 'ally' can't trust you, why would they have any reason to trust you?

It seems that your strategy (if you can call it that) works because people actually trust you. That won't make you many friends and it won't win you many games.