There are only 2 ways to start a ground battle, listed below are the 2 ways.
1. Sending an attack fleet to a planet with ground units in.
2. Sending a fleet with ground units in, to invade a colonised planet.
There is a huge difference between the two orders.
The Attack order will allow you to attack a planet but not take it over with a ground attack, you can only destroy any defence on the planet surface.
The Invade order will allow a ground attack to destroy any defending units or PDS and if successful, it will give you control over the planet and any fleets,
units or buildings it still controls.
Please Note - Only 3 of the 7 PDS will attack hostile ground units.