"Aggressive tribes try to exterminate the competition. They ruthlessly attack and hoard all the resources for themselves."
Gadget Bomb: "This ability causes damage to buildings and vehicles and can help capture cities."
ICBM: "This ability captures all other cities by military means. You also get the Conclusion achievement."
Arms Dealer: "This ability gives you discounts on all combat tool trades."
The aggressive trait is gained by destroying competing tribes.
If a tribe is too difficult to defeat, try waiting till some of the enemy tribe members leave their village to go gathering, fishing,etc. Once they are far from their village, send all your members to attack them. Repeat until you get them down to 4 or 5 members. they will be an easier target.