The Inca special unit is a warrior that gets +100% vs archery units. If you really want an early war at the beginning of the game, there is none better. No research required. Just build and attack. As long as you find the enemy cities before they have axemen, you'll do fine.
In Noble and up this is not possible. I tried the Incas on Prince and I was able to wipe out the Japanese who had only a bunch of warriors and their capital was my biggest city later on. Then I tried on Noble againts Celts (Budica? I don't remember the spelling), she had four archers when I got there. After burning 12 quechuas I gave up asking for peace

