I've seen tons of pros in 1v1 ladder games attack each other with 1 army but I've never really been able to make out why.
So far I only understand two benefits:
1) The opponent plans to attack a neutral with 3 armies and only has a total of 4 armies on their territory. If they lose 1 army from the attack, they'll run 2 armies into the neutral which can cost them a lot of momentum.
2) The opponent is about to strike your territory of 2 armies. He would kill 2 armies while you would only kill 1 army. However, if you send an attack of 1 army, you'll be able to kill 2 of his armies as opposed to 1.
But then you have these games where Tyrion is constantly attacking with armies of 1 regardless: https://www.warlight.net/MultiPlayer?GameID=10712556
(Turn 2 in Central Russia, Turn 5 and Turn 10 in Central America)
Like... wtf can someone please enlighten me on this.