First it is best to contact the leader or moderator of the Alliance you wish to challenge.

Then, go to the profile of that Alliance. There are multiple ways to do this but one is that you can look them up on the rankings ladder for Alliances.
(Try searching their name).
If you don't find it this way, then you can try getting there from the profile of a member of their Alliance.

Once you are on their profile page you will see a box that says "Challenge".

Once you click this, it takes you to a set up similar to an Internal game.
You may now set the conditions for your challenge game and issue it.

The other side will have to accept it.
Make sure you set it up right.
I recommend using random nation selection.
Set the number of total players you want in the game. For example: for a 5 vs 5 you will want 10.

If you feel that you are ready for a tough challenge, The Last Legion would love to receive your challenge.
Or message me in the game any time and I can recommend a few Alliances to you that you can challenge for an even fight.

Which Alliance are you btw?

Oh and btw you have to be a moderator or leader to issue a challenge.

hello george kenneth how many players do u have and we should talk and get together on a alliance game

I know of two other Alliances who are active and are now accepting challenges from equal opponents:

Team Awesome
International Terrorist Assault Group

And if you win those games, I suggest you try:
French Legion
The Last Legion

yes cyclone46 i got your message and the tow alliance is still debating that part of the game so hopefully those teams will be getting a communication from me soon

okay Tribe of wolfmen is currently 15 strong with most everyone active hope to see u join for the alliance event then as we will have a 5 person team there