Army Internist here. I'll echo what everyone else said. If you want to be a hospitalist, the military makes it tough. I work at a MEDCEN and our hospitalists are civilians with military staffing the wards 1-2x per year. It's not quite as bad as what HighPriest said, the average age on my census is 60-70, but the census is low and they are not as sick as the patients down the street at the civilian hospital. We take care of active duty, retirees and their families. Those folks tend not to be homeless or addicted to hard drugs for years without treatment. Additionally, they have access to healthcare by virtue of being able to be seen at your facility so their A1C isn't 13 (usually.) You'll occasionally see someone transferred from the VA who has those issues, but your census will be filled with mostly uncomplicated HF or COPD exacerbations, PNA, a touch of the sepsis or osteomyelitis waiting on a PICC or someone to take their toe off. Before COVID you could moonlight if your command allowed it, which is where people keep their skillset active.
The reality is you'll probably get stuck in a clinic somewhere. If you're lucky you'll be at a MEDCEN with the potential to do wards 2-4 weeks a year (everyone wants these shifts.) If you deploy, unless you're with a CSH its all outpatient. Mostly MSK and psych for 20 year olds. You'll occasionally catch something interesting like renal failure from someone taking 2 grams of Advil 6 times a day, but it's not intellectually stimulating. On the plus side, if you need a test, treatment or specialist, you can get it for your patients. It's a good thing these folks are healthy, but healthy patients don't make good doctors. You need a census full of poorly controlled diabetics with coronary arteries you can map out on a dry CT and severe COPD to stay at the top of your game. Best case scenario you get an understanding command, moonlight as much as possible and get out in 4 years. You can maintain your skillset but you'll have to work at it. As far as this affecting your choice in specialty, thats MILMED in a nutshell. Whether you're a spine surgeon or an Internist, you'll need to moonlight to keep up. The upside is every nearly every hospital needs hospitalists so the opportunities to moonlight are pretty good without having to travel across the country. Admitting shifts in my area get about 1800 for a 12 hr shift, more for nights.