You should start an alliance when you are sure that you and your partner(s) are strong enough to kill everyone else.

Or when you and other players see a common threat growing.

i say round 6 truces are the best.

round zero!

first!

i prefer pre game alliances ;)

The best round to start allying someone is the round before someone else tries to. Making the first alliance is important.

It's also important to ask for the alliance when it's about to be YOUR turn or your potential partners turn. This gives you the advantage of surprise and allows your alliance to make the first attack on others.

The second best time is immediately _after_ two other players have truced, or given any indication that they may truce.

Counters are often more powerful than the original truce (whether it existed or not) and organizing a counter-counter is quite difficult.

I agree with Verm, counters seem to be the most effective due to the fact there are usually more than 2 participants.

round 3-5