Logically, Lieutenant MacGregor would have known that the chances of him holding a Company command position for a mobilization to Iraq were slim, but he had his hopes.
Remember, that the most junior of all officers in the US Army is the “Second Lieutenant”, or 2LT. As long as that 2nd Lieutenant doesn’t do something egregiously stupid or outright criminal, he or she will probably see First Lieutenant, or 1LT rank, within a few months. Some are late bloomers, sure, but I’m talking in generalities here.
Lieutenants, both First- and Second-, are put in charge of Platoons, which are about 30 or so people (it can vary in size depending on the unit and its expected mission). There are usually four Platoons in a Company, so there should be four Lieutenants, and the senior-most one of them is usually the XO, or Executive Officer, that helps the Captain and serves as second in command.
But a lot of Reserve and National Guard units don’t have all the people in them that they are authorized to have. It is not uncommon for units to be lacking people that would be considered critical for daily operations, and I have been in many units where platoons did not have any lieutenants assigned to them and senior Sergeants ran them. Also, there may not be a Captain in command, so the Lieutenant takes command.
This isn’t just an officer situation, either– I’ve been in units that didn’t have First Sergeants or other leaders, and some that were so under-strength that while they were authorized four Platoons, they just had enough personnel for two, so that was what they ran with. This is why, when units get called up for mobilization, these under-strength units have to get “fills” from other units to bring them up to proper deployable strength.
Lieutenant MacGregor was hoping to be uncontested for command, so he would be readily promoted to the rank of Captain. While there are other officers being assigned to the unit, he outranked them or had them beat by seniority. However, a mysterious Captain has also been assigned to the unit, so his hopes are dashed.