The sea is used as an analogy in space travel, and sometimes it works well. Ships sail on long journeys, between distant lands, across a hostile medium.

But these analogies tend to break down when it comes to planetary invasion. Planets aren't like islands: because an island has a coast that can be defended, and defenders can retreat into the interior of the island. This happened a lot in WW2, particularly in the Pacific. But in terms of planetary invasion... the entire planet surface is the coast, and the interior of the planet (the mantle) is not something which can really be retreated into.

Much more important, bridgeheads do not seem to be important. Spaceships can land virtually anywhere there is a flat surface on a planet. The concept of a front line in terms of planetary invasion seems somewhat irrelevant if you can land an army behind what is the "front" line.

Clearly there must be means to counter spaceships. Spaceships when in close orbit, or in the atmosphere, become subject to the same rules as aircraft. They can be hit by terrestrial missiles and other vessels. But all this means is that air superiority be achieved before invasion, and defences hit from long distance by bombardment.

Which brings me to the last point: why invade at all to destroy an enemy? Most structures of a civilisation are probably going to be on a surface. It is, after all, more difficult to build underground than on a surface. That means that most items of value can be simply blasted from above. Sure, some command points, armament storage and production facilities, and offices of government may be transferred to secure underground bunkers, but this still means essentially conceding the surface to whatever punishment the attacker chooses. There can be no counterstrike, merely a populace putting its heads between its legs and hoping for the best. Perhaps in the event of a siege there could be hope of external assistance, but this still makes it such that the attacked planet is itself left defenseless.

So is there any way for realistic planetary invasion have any meaningful strategy? I have never seen such in fiction - the invasion just "happens" and we generally catch up with the consequences of the invasion. If the invasion is defeated, it is always some Deus Ex Machina that has nothing to do with strategy relating to the invasion itself.

Edit: By "invade a planet" I mean from a strategic, military point of view. The Allied invasion of France during WW2, for instance, wasn't just an exercise to liberate western Europe - the Axis forces needed to be eliminated on the ground (aircraft not used in combined arms offensives were of limited use except in urban bombing, and urban bombing had no significant effect until airbases within mainland Europe were available). The option to float thousands of gunships a mile up to rain down precise fire 24 hours a day simply wasn't available, even when looking at recent historical wars. Had it been available in something like WW2, there simply would have been no Axis forces left to resist invasion.

Edit 2: I should probably have defined the parameters of this question as EITHER "Why would an interstellar civilisation invade instead of attacking a planet" OR "Given that an interstellar civilisation is invading a planet, how would meaningful defence be mounted?". I was kind of leaning in favour of the latter definition of the question, but that's a damn difficult question to answer (if it wasn't we would have seen it in sci-fi settings already).