why have 2 ask commander if you can marry?

Hollywood Fiction: he didn't have to ask to marry her because they were already married. And as depicted in the movie, she wasn't calling to check in on Bernie from the switchboard because she was home with the flu for the duration of the storm. For the most part, the role she plays in the film is fabricated presumably because shots of her sniffling in bed would have been much less interesting to watch.

It used to be standard for all American military branches. As past wars were fought overseas, there were concerns about men marrying foreign women and those women then being denied entry into the US. It was thought that a man marrying in the midst of war, then being shipped somewhere else and having to leave his new wife behind could be bad for morale, and distract him from his daily duties. There are also statistics that show an extremely high divorce rate for these overseas marriages.

Additionally, soldiers used to be granted leave specifically for marriage, which requires permission (as any leave does); and overseas marriages required lots of documentation and paperwork so that the marriage would be assured to be valid when the war bride came into the US.

Nowadays, it's more of a formality, as stated in the film. But newly married military personnel do still have to officially provide updates of new last names, new addresses, changes in insurance, requests for housing for their spouse, etc., after they get married.

I didn't necessarily have to ask for permission, but when I was in the Army and got married, I had to notify my platoon sergeant, who had to notify our commander. (I joined the Army in 2001 and got married in 2003.) This was done in order to start housing allowance, get your spouse registered for an ID and medical care on base and get them added to life insurance. The weirdest part, and I'm not sure if this was done everywhere or just in my command, was that I had to bring my platoon sergeant to inspect the apartment I was moving into because there was no on-base housing for E-4 and below and I was an E-3. So if you had dependents (spouse, kids, etc.) you had to have someone OK the place you were moving into. Before I signed it, I also had to bring in my lease to ensure it included a "military clause" which states that if you're deployed before the lease is up, you aren't responsible for the remainder of the lease. Although I think nowadays as long as you have orders, you're covered under the SCRA, so you wouldn't even need a military clause.