I'm not a Visser Three (V3) fan.
|Image from animorphs.wikia.|
I think the Earth Invasion should be credited to Visser One alone. Yes, V3 infested an Andalite, but I believe he did many things that a politician would do to hold power, just to be the one to infest an Andalite. He's not a thinker, and to lead a war, one must think 3 steps ahead of the enemy, just the way Jake did. V3 did not do that.
Also, it takes a leader to lead, and V3 is no leader. He easily kills his subjects when he's annoyed, even the most loyal ones. And he's easily annoyed. His subjects are afraid to be killed; they're not afraid to disobey or disappoint him.
Because of his true nature - angry, impatient, impulisve, vengeful - he wouldn't win any lengthy wars, for sure.