If they don't succeed, they don't cause ANY damage to the invaded planet. So when you have more troop transports and face a planet with large population, you can combine them: with initial troop tansport(s) you use the most destructive method (usually Mass drivers) to significantly increase your soldiering bonus and remove large chunk of defending population. Then you proceed with less destructive tactic, to reduce the chance of destroying buildings or decreasing planet's PQ.
In Dark Avatar and Twilight of the Arnor you can decrease population on a planet by planting spies on all farms. In the next turn the planet's population will be reduced to 6B (to 8B in ToA)(if you're invading a colony). Caveat: those spies may be neutralized, but pop will still be reduced.
In Dark Avatar and Twilight of the Arnor you can decrease approval on a planet by planting spies on all morale buildings. Then you need to select another planet, and select back the one with spies. Approval on it will be severely reduced, so you can use Information Warfare with greater chance of success. You can re-use those spies on another planet in the same turn.
Description: Using the troops on hand and the best military equipment available, we send our soldiers into battle the classic way. Our men won't receive any bonuses, but this is the safest way to attack without fear of damaging the worlds we are invading.
Note: the only one that's free. Use it when invading low-population planet, or your soldiering advantage is large.
Description: By attaching drive-thrusters to nearby asteroids, we'll be able to use the space boulders as large-scale projectile weapons. While the attack will be almost unstoppable, destruction on a planet-wide level is inevitable.
Description: Before the attack begins, we'll send a crew down to the surface of the targeted planet. The group will equip a half-dozen mining vehicles with Illudium Lite explosives and direct them at the core of each planet. This will cause a planet-shattering explosion that should significantly weaken the enemy defenders.
Sort of a halfway point between Mass Drivers and Tidal Disruption, except it costs slightly more. No real reason to use this.
Side note: Illudium Phosdex, according to Duck Dodgers of the 24th and 1/2 century (Daffy Duck), is the secret ingredient of shaving cream. On his quest to get more, he met Marvin the Martian who in another episode attempted to blow up our planet. Whe Bugs thwarted his attempt, Marvin was quoted as saying: "Where's the Kaboom? There was supposed to be an Earth-Shattering-Kaboom." I wonder if the text and the Loony Tunes connection are more than coincidence?
Description: By scattering gravity-altering satellites into orbit around inhabited planets, we'll be able to control the oceans and bury our enemy in a watery grave. Monsoons across the globe will greatly decrease enemy defenses, but will also destroy almost all planetary improvements.
Description: Through the miracle of advanced artificial intelligence, we are now able to send smart robots onto the battlefield. These fighters pack quite a punch, but can still be taken out with relative ease and are fairly expensive to produce. They are also slightly reckless when it comes to close quarters fighting, so some improvement loss is to be expected.
Description: Preceding attacks, viral clouds will be released upon the most populous areas of the target world. Our enemies defensive strength will be weakened, but irreversible damage to organic life on the planet will hurt overall planetary quality.
Description: Communication vessels orbiting inhabited planets will warn citizens of our invasion plans. By promising them a better life under our rule, unhappy citizens are likely to join our side. However, for proper distribution of our message, the price will be high.
Cost: 800 BC
Converted Troops: 0.2-0.4% planetary population for each percentage of unhappiness on that planet.
Use it when facing a planet with large population and low (below ~65%) approval. If you have significant advantage in soldiering, you may end with more pop than you've brought troops.