I was in the army for 5 years but was irreligious at the time. I will try leave politics and personal bias out of it and I will try to answer your questions about military life, based on personal experience, from a muslim perspective.
To get right to the big question about killing other muslims:
Ask yourself, how many times did Saladdin raise arms against fellow muslims? How about Abd al-Rahman? This guy? I'll tell you right now, having witnessed firsthand what crimes the muslims I fought against were committing against Afghan populace (who are also muslim, btw) I don't regret raising arms against them in the slightest. Whatever justification they come up with for the atrocities they commit (US foreign policy, Israel), I can't honestly say I feel I did the wrong thing by taking part in an armed struggle against them. And honestly, with the troop withdrawal and everything, ambitious soldiers will be fighting each other over the opportunity to go overseas (looks real good on promotion board). It was already like that when I got out a few years ago, so I'd say your chances of taking part in any fighting now is near zero.
On to military life:
Prayer is going to depend greatly on situation and environment. If you're in basic training and in the middle of a 10 mile ruck march, you wont be stopping for fajr. If you're doing PT with your squad at your duty station and you ask your squad leader to stop for prayer, they will probably grant it (if they don't, you can take the issue up with the commander or higher authority. Believe it or not, religious freedom and diversity is a big deal in the US military). Same story if you're at work in the middle of the day and ask to step out for prayer.
There are muslim chaplains and islamic services offered. If your base doesn't have one the residing chaplain is obligated to direct you to the nearest masjid off-base.
Jumah could possibly be an issue. With the exception of long shift work, the army operates on the normal federal work week, which is monday-friday. Whether you are allowed time to attend jumah is completely up to the local command structure.
With the exception of during basic training (where consumption of food and water is compulsory) it is entirely possible to observe the ramadan fast while serving in the military. I personally witnessed a muslim soldier have a physical fitness test deferred to the next month so that their fasting would not impact performance.
Dress code and grooming:
This is the part where military regulation completely trumps religious practice. If you are a man, kiss that beard goodbye. During the summer months, females will wear shorts with exposed legs during physical training, which is done alongside males. The dress uniform for females looks like this. I honestly don't know what the regs say about hijab but I think a woman that wears hijab will already be turned off by shorts and skirts.
And don't count on making hajj during your service. International travel is pretty restricted while you are actively serving.
Lastly, whatever you do, remember that nobody, not the brothers and sisters who think they know better than you, not your family members, nobody can get between you and the Greatest. Follow what is in your heart, and worry about what will please Allah (swt) and not what will please any of us.
Hope I helped! Feel free to PM me if you have any other questions.