Obviously we need to clear the enemy off of this rock, so that we can bring in the engineers to strap a bunch of engines to it and ram it into that rock with all the water and such! 

"Hey, Bob?"
"Yeah?"
"That planet we're supposed to be going to - wasn't it supposed to have... I dunno, water and stuff?"
"Yeah, so?"
"Well, we're headed towards a barren rock with no atmosphere... that can't be right, can it?"
"You go to the navigation coordinates they set."
"But..."
"The brass says we head to THIS rock, then we're headed to THIS rock. They know better than you."

I`m engaging an enemy unit. At first the battle went well and we inflicted heavy losses.
Now they just sit there, they still have supply and a separate supply batallion for both.

https://imgur.com/rUfJWKH

I see a planet with an active sensor ring around it and a troop transport group. I do not however see an enemy ground signature. Are you sure there are any enemies left? And if yes, are you certain that the alien race is set to hostile?

In case the planet I'm referring to is not the planet where the combat is happening, please show a screenshot of the combat zone to see if there's anything else that would stop combat.

Your formations are set to frontline attack so if there are hostiles to fight they will engage. However if the enemy is not set to hostile, they will only return fire when they themselves get attacked and will not continue fighting without provocation.

Edit: Also be aware that if the fighting has destroyed all installations of value in a population, there is a chance that the fighting will end without you conquering the colony (this assumes that there are no minerals or missile stockpiles present), the associated ground combat event should tell you if this is the case.