Leaders and initiative are the two central concepts of this game, important to a degree not seen in other simulations.
Leaders may be developed. Although there is a provision to raise their offense or defense rating, unless it is a "1" to start with, it is unlikely that you'll ever see this happen.
Administration ratings (important for a TC) are not subject to improvement.
Rank and command rating, both particularly important for TC initiative calculation, can improve.
Some leaders get promoted automatically on a certain date. Although I've never done the analysis, I suspect that their promotion date is objective, meaning they get promoted that month regardless of the turn they enter play even though the game reflects the promotion date in the "relative" time frame, meaning a in the future in a certain number of turns.
Leaders gain points toward promotion through successes, typically commanding battles or training troops (engineers and cavalry have special rules).
Command rating may rise/fall in some circumstances, anecdotally to my recollection typically increases/decreases of 1 or maybe 2 depending on performance in and outcome of battles.
Which brings me to General Butler. This political general was militarily ineffective in almost everything he was assigned, yet remained in command through the end of the war. He is important to a Union vicotry, and here is why.
On turn one, move him from Fort Monroe and make him an army commander. As you look at your candidates, you don't have many that qualify for that post anyway, so go ahead. He will always be your worst army commander in terms of performance, but ....
1. During the time until you have a real army with effective leaders (in summer/fall 1862), Butler will continue to contribute his 4 PPs rating every turn. Butler serving from August 61 through July 62 is 4x12 = 48 political points, a greater contribution than the capture of any one CSA region except Richmond.
2. He starts being an AC with 2 stars and 230 promotion points, more than enough promotion points to qualify for three stars. Give him things to do that generate successes. Although he is a poor trainer, give him some militia. Every training success prompts a promotion check (remember, he already has enough points, he just needs to generate more promotion checks to get that third star). As I recall, he can build depots and forts and get successes too. Eventually, Butler will get his third star.
3. With a third star, now look at Butler as the next TC (this is truly the 70's Peter Principle in action). Winfield Scott typically dies by mid 1862 and Fremont's a TC by virtue of starting the game as such (and the only other candidates are McClellan and Halleck). By promotion to AC and then TC plus tangential participation in some successful operations Butler's CR will be 20. He has 3 stars. His admin is 4. Therefore his chance of getting initiative is 20/20 x 3/4 x 4/6 = 50%. If 50% is not great, it is about as good as it gets for the Union (McClellan is likewise for example 20/20 x 3/4 x 4/6).
In the forums many years ago there were discussions on trying to develop the Union leader Dix to be a TC. If he earned a third star he would have an initiative die roll of 20/20 x 3/4 x 6/6 = 75%, very formidable. He has many chances for success; in addition to being a 6 for building forts and depots, he trains at "4" for infantry. Nonetheless, and despite my consistent effort, I've never seen him promote to three stars. If anybody has, please let us all know.
"L'audace, l'audace, toujours l'audace."