Back when we (A&K) were 4th-5th and in NW only, B-A (Black Arrow) were in the same area. We were allied with them, then the alliance grew sour somewhat and we went to war. They're still around, but weak and slowly being picked apart.
T.P (The Plague) was a very aggressive tribe, long-term no.1-2 and at war with pretty much everyone. We and T&U (at that time no.1-2 as well, we were no.3) were allied and fighting them successfully; in the end, their leadership gave up, so we merged into them, but our leadership took over and we internally nobled a lot of their players that already gave up. At about the same time as the merge (idk exactly when), we found out T&U wanted to fight us.
However, T&U had a lot of problems after their leadership went afk for a week or two and the tribe wasn't able to coordinate against us at all, it was totally in ruin; some of their players went over to us, others to other tribes, and we kept fighting those. Also Legion was invading our provinces and later attacked, so we went to war with them too.
I don't think there have been too many merges - WUT merged into T.P, at some point and then we did and renamed it to IMP, but tribes have broken up too.