Carl takes time off from his duties in the middle of a very important operation to send Rico a psychic message that helps him rescue Carmen despite only having seen them once since they left for the military and it wasn't under great circumstances.
The little speech Carl gives about how things are finally starting to look up in the war.
Carl: We've got one of their Brains now. Pretty soon we'll know how they think, and then we'll know how to beat them. One day it will all be over, and everyone will forget that this was the moment. This is when it turned. And it wasn't the mighty fleet, it wasn't a fancy new weapon. It was a drill instructor named Zim, who captured a Brain.
After Eight Chapters of not even allowing his wife to talk about him, much less write to him since his enlistment in the military, Johnny's father reunites with his son as a Soldier Himself in the Mobile Infantry in a big, tearful embrace of pride and love.
Just before Rico goes on his OCS practical assignment as a 3rd Lieutenant, the OCS Commandant informs him that Lt. Colonel DuBois, Rico's high school teacher, had written to him and asked that the pips he wore be given to Rico. Rico is touched, especially as he and DuBois had never gotten along in school... until the Commandant regretfully informs him that the pips are unavailable due to a real estate deal.
He's then offered a set of unlucky pips, where the last seventeen cadets to wear those pips have flunked out. He swallows, accepts the offer to try and break the bad luck curse... and the Commandant thanks him, and tells him that these were the pips he wore as a 3rd Lieutenant.
Towards the end of the book, Rico is on his OCS practical assignment as a 3rd Lieutenant, leading an MI section into a Bug nest. His Platoon Sergeant is missing. Torn between leading his section to look for his man, and continuing on mission, Rico orders the Section Leader to continue the mission while he goes alone to make a one-man search. As he drops down into a hole and starts walking alone, he hears the Section Leader taking roll of the men on the radio. And then he hears the last order: "Section! By squads! Follow me!" And then the entire section jumps in after Rico.
This is heartwarming because the rank of 3rd Lieutenant is a provisional rank, for OCS cadets who are not yet commissioned, but are being trialled. That is to say, they have no independent authority whatsoever. The squad followed him purely out of personal loyalty.
Not quite correct. The Commandant explains that cadets are given the rank of 3rd LT so they do have a place in the chain of command — and the responsibility that goes with it.