Watching the Halo TV series. So in the battle for Reach, somehow a dimensional portal opens up and the MI and Fleet that attacked Klendathu from the book, the second time (NOT the movie) appear over Reach and join the fight.

Is reach successfully defended? We really don't know much about Fleet capacities from the book for ship to ship combat. They do have planet busting bombs.

The suited up Mobile Infantry had history fighting swarming enemy infantry with energy weapons, slug throwers, etc,. This the book! All of them have some small tactical nukes.