Can a 2 yr RN become an officer in the US Army?

You cannot become and officer in any branch of the military with out a bachelors degree. So you would need to get your BSN before you could even think of becoming an officer.

Not Active Duty, but maybe in the Reserves, though the Reserves has begun changing the policy of no longer granting commissions to ADNs, and instead are now in favor of BSNs. Since you don't have your degree yet, my thought is that by the time you have your degree, the Army (Reserves and Active Duty) will have completely changed the policy to BSNs only.

No other branch grants commissions to ADNs at all. The Army is following this trend. There is not as critical a shortage of Nurses in the military as was in times passed, so every branch can afford to be a bit more selective in to whom they grant a commission.

You'll need the four-year degree to commission straight off of the street if you want to go active:

http://www.goarmy.com/amedd/nurse/corps_specialtie...

An option you have is to enter enlisted and using the AMEDD Enlisted Commissioning Program listed at the bottom of the page to get your bachelor's so you can get commissioned. You'll earn your degree on the Army's dime and get full pay and benefits while doing so. A two year degree will allow you to enter initially as an E-3.

If you are willing to go Reserves you can use a two-year degree:

http://www.goarmy.com/JobDetail.do?id=315#requirem...

However, Reserves get reduced benefits compared to active duty. But, you can always work on your education and then switch to active at a later date. It's up to you.

No you have to have a 4 year degree.

sure, yet they could desire to earn a school degree first. maximum generals start up as 2d Lt's after college. yet there have been generals that have began as privates Earned a school degree, been commissioned and then rose to regularly used.

you must have the BSN in hand before you qualify for a commission as a Nurse.

No, you must have the four year bachelors degree.