When I say planetary assaults, I meant attacking an alien empire's homeworld with the intent to crush their fleet and PDCs, then force the population to surrender either by landing immense numbers of ground forces or by bombarding the planet with nuclear missiles. The latter is easiest, the former is the more profitable.
Hostile interactions with the NPR's in the game usually begin with a scout ship or two being destroyed when they stumble across a colony or home system.
The next phase tends to be sending a fleet in to see what is there, if it is a young colony world there may not be many defenses, in this case you are likely to take it over and get some information on the other race, some idea of its technology level and perhaps even some of the layout of the empire, you get this by picking up escape pods, forcing colonies to surrender, or by landing espionage teams on a colony and having them try to steal info.
However, if you run into a strongly held system, your fleet will likely meet a premature demise or be forced to retreat depending on your tech level and numbers. Most NPRs devote tons of resources to their space fleet, and fighting 50 or more ships at once might not be unusual.
So there follows a period of fleet building as you adjust your technologies and ship loadouts as well as increasing your fleet size. The next incursion into their system you are likely to win, but now you have the dilemma of what to do with the inhabited planets and civilian fleet, which will annoy you by moving in and out of sensor range and making your time increments not advance properly.
The worlds still have shipyards, and will be producing military vessels as fast as they can, you can destroy the shipyards if you like, but they take a good chunk of firepower and you might want to capture them instead, this requires ground forces.
Lots of ground forces, the higher the population, the harder it is to force a surrender, to say nothing of the ground forces the NPR might possess.
You can reduce the ground forces needed by reducing the population via orbital bombardment. But that will also damage planetside industry and raise the radiation and dust levels on the planet, also if they build a ship or PDC during this time they can and will shoot missiles at your ground forces, making you wait even longer and tempting you to destroy the shipyards.
Once you win, then you get the races technologies, most or all of their existing ships, full knowledge of their surveyed systems, and everything on the planet as well as their shipyards.
And all it cost you was a few in game years of work, and hours of micromanaging space combat! Mind you I find it extremely satisfying to look at the combat log and see "ship takes 8 damage from your awesome missile of awesome, ship is streaming atmosphere, ships thermal signature has changed from a lot to less, ship has been destroyed" And so on.
More than likely as well is if the NPR has been around for a bit they have probably met some other aliens too, and might even be fighting a war with them! Putting you in the position of figuring out where yet another empire is and getting defenses in place.
