You're attacking an enemy planet with troops! Against entrenched troops! And my people have antimatter warheads, which you don't. 

I mean, you don't need to go into atmo to fire orbital ordnance. You can sit  as far away as Mars from Earth, and still hit something. Probably won't be accurate, but you could still hit. You came in [relative] very close range of antimatter missiles. The Abh move with antimatter missiles were outdated and simple ICBMs. These are big weapons specifically meant to kill enemy ships in orbit. That is what Draconians do- they kill soldiers, they kill more soldiers, and then they entrench themselves and kill anything in range- on the ground, in the air, or in the orbit above. 

And there would be something stopping you from nuking the planet, though if you were determined enough, you could do it. The radiation might be so high that even environmental shields might not be able to take it out, but I suppose you could go with the idea of if-I-can't-have-it-you-can't-have-it.

But then, if you nuke my planet, then my people will do the exact same right back. 

If you want advantages, try, I don't know, sending down some specifically-stealthy scouts to mark targets, use orbital bombardment to clear out entrenched spots, and then drop. But you didn't. You just... rushed everything. You have an advantage in numbers, but how good are numbers when you're sitting in an open field, coming down bit by bit, and getting shot at all the while?

What comes around goes around. Invading my planet will probably get me invading yours. Trying to use chemwar on my planet will probably get me using chemwar on yours. Trying to nuke the entire planet into submission will probably get me using nukes on yours. 

You have the advantages of numbers and orbital superiority. The problem is that on the ground, your steady stream of numbers probably won't be enough to win. And in orbit, you'll be pummeled by ground defenses the entire time. Including antimatter missiles. If you have powerful shields, you probably won't die so easily, but armor is material, and antimatter is pretty much antimaterial. It is THE antimaterial. That's why it's antimatter. 

The only thing is that you're invading Draconians. The Race who Conquers doesn't take kindly to attempts at conquering. If someone has the gall to invade, the Draconians are both going to think, "WORTHY FOE OF MY BLADE" and "HOW DARE THEY EVEN THINK ABOUT INVADING US!"

You're invading the warriors. The WARRIORs. Now, I may or may not have fired first shot, but Draconians have little interest being trade partners, and they have little interest in threats of annexation, so it's pretty much a pre-destined thing. 

Note that I'm not raging. I'm actually pretty amused.