Thanks. Planetary Defense must be the key. I have yet to see/invade a planet with a non-zero planetary defense, so invasions have been frightfully easy so far. I'm playing on normal difficulty, so maybe I need to kick it up a notch to see some more fight from the AI.
Edit - Seems it's more complicated than I first thought. Found http://galciv3.gamepedia.com/Ground_invasions which is supposedly up-to-date with the version 1.01 patch. It explains, among other things....
"Defending: A planet's defense is determined by 3 things:
- The population of a planet is divided by its class, so a class 10 planet with 20 pop, ends up with 2 defending troops per tile.
- Resistance is the % of that population that will fight, if the planet has a 50% resistance, then each tile will have 1 defender.
- Planetary Defense is the most straightforward: a percentage of the invading troops are killed immediately before landing on the planet surface. If 6 soldiers try to invade a planet with 50% planetary defense, half of them die immediately. 6 Invading Soldiers becomes 3.
...The invading troops that made it through the Planetary defense system, land and take out one tile at a time."
So in my case:
1. population 16.2 / class 15 = 1.08 defending troops per tile.
2. 1.08 * resistance .51 = .55 gives .55 defenders per tile that will fight.
3. No defense, so all 4 of my attackers make it to the surface.
So 4 attackers are fighting 0.55 defenders over each tile. Unfortunately, the article doesn't explain how these individual tile battles are resolved.