Can someone please explain how invasion success/failure is calculated?

Not sure either, but as you note, from the math it's most likely that 0% defense.  Which could make sense-- as having 8.3 million people who are willing to fight but have no stored weapons to do so versus 5.2 million well armed soldiers is very likely only going to end one way.  Still, probably needs to be tweaked in that even though they wouldn't win, the civvies could still cause a decent amount of casualties.

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:

1. 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.
2. Resistance is the % of that population that will fight, if the planet has a 50% resistance, then each tile will have 1 defender.
3. 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.

The Vietcong were wiped out as a fighting force in the Tet Offensive of 1968, which was a combined Vietcong-North Vietnamese Army operation.  They were unable to field any unit larger than a company for the rest of the war.  The North Vietnamese Army launched another major offensive in 1972, involving multiple divisions with tanks, which was again beaten back with heavy casualties.  General Giap (North Vietnamese Army commander) wrote in his memoirs that it took another 3 years to raise enough 16, 17, and 18 year olds to replenish the army and attack again in 1975, by which time the South Vietnamese army was fighting on its own without even any U.S. air support.

Vietnam has really no point of comparison in this.

Asymmetric conflict is not modeled in the planetary invasion, or post invasion simulation. The invasion models a conventional war.

They still won. 

Osbot, there is a difference between being on the winning side and winning. Just ask Poland, or more specifically the Polish faction more closely associated with the pre-WWII Polish government, how being on the winning side went for it after WWII.

North Vietnam won the Vietnam War. The Viet Cong were more or less a non-entity from the aftermath of the Tet Offensive to the end of the war.

One question: Can resistance ever go above 100%?

If by "They" you mean the Viet Cong, you are dead wrong.  The North Vietnamese regular army defeated the South Vietnamese army after the U.S. pulled out of the war.  The poster who claimed that untrained farmers defeated a bigger and better trained army was incorrect and ignorant in many ways.

Your previous post about that conflict not being a model for planetary invasions was much more cogent.

Yes, but I'm not sure whether it's supposed to.  A Preparedness Center improvement, boosted by adjacency effects, can cause resistance to go above 100%.  There's an acknowledged bug that the level increase for Preparedness Center is 95% when it should be 5%, but I don't know if there's supposed to be a planetary cap at 100%.

Resistance > 100% confirmed in my game, like Publius of NV states.  Preparedness Center in a triangle with Planetary Defense System and Military Academy will do it. High Approval also adds to resistance -- looks like 100% Approval provides +25% resistance.

Tbh, invasions are way too easy to pull off.

It makes little sense to me, that you can just pull 3 pop off a planet and take another planet without much trouble, as long as no ships are defending.

To me, resistance in current form don't make much  logic sense. You ultimately can pull 99% of planet populace to attack planets (load them on transport) but if exact same planet is invaded, only a percentage of populace is combat ready? And no, we don't train invasion force, we just load then on transport where are shiny military stuff is.

You can defend a Planet atm, i play campain Lev 2 and with 3 Transport i get 0% strength, the defense takes 100% down....

No Chance to invade.

Resistance can and is supposed to go above 100%. The preparedness centre is bugged should be fixed soon, if not all ready. The bonus is allowed to go above 100% because it can be reduced by attacker modifiers. I don't know if this actually allows you to have more population defending than your planet has, ie. they're more effective than one pop point normally would be, or if it caps it at 100% effectiveness after invasion modifiers have been applied.

Yes but they lost every major battle. The war was not lost in Vietnam it was lost here at home due to the Hippy movement among other things.  So you could say that the Happiness/moral factor was very low and this was why we lost.  Now if this were GS3 and I was the leader then I would have taken the Malevolent approach and shipped all the damn hippies to an Island in the middle of the Pacific and tested some of my new nuclear missiles on it.   That would at least be worth 10 points to my Malevolent Ideology score.  I could get a free ship.

This fascinating though the discussion of Vietnam is, doesn't resistance actually effect how likely a planet is to resist culture flip rather than invasion? I would love to know though how defense is worked out in planetary invasion, I also think they need to do a lot more work to make that aspect of the game more interesting. I too have never failed to invade a planet.

Yes, resistance does have a major effect on culture flip chances in 1.01. Not sure what the formula is, but I think it was corrected a little too far in favour of resistance.

Also, can you Vietnam flame-war-people go create a thread in the off topic area? I don't need RL war and politics in my GSIII threads. Reference fine, debate for ten posts, gtfo.