What should I branch before becoming a Civil Affairs officer?

I'll second COL (Join to see) and give you some of the whys. Think about what you need to bring to the table. Understanding the Army mission, organization, etc. is essential. Knowing what the Army does well. A good CA knows how to relate in country to the locals and leverages the best aspects of the Army capability. The Army is in the war fighting business first and foremost. You'll want to get some of that been there, done that under your belt; sooner the better. Be that Platoon Leader. Get as much field experience as you can. If you spend your first two tours being a glorified IT Wonk, it's a steeper hill to climb.

That said, give your all to the first two tours. You may fall in love with what you're doing or something else. As no plan survives contact with the enemy, neither will your plan for supposed future bliss. The world will throw many a curve ball at you. And, take a cue from our Marines; Semper Gumby.

I became a Civil Affairs Officer after being in Military Police and taking the M.P. Officer Basic Course or Basic Officer Leadership Course as it's called now. Since I was in the National Guard, I was able to Branch transfer to Armor and I had 5 years experience and one deployment to Iraq with an Armored Brigade.

The other comments here are correct. Taking whatever Branch you desire, possibly Combat Arms, can help you work better with other Combat Arms personnel because you will know the vernacular. Not sure if Civil Affairs can be Basic Branch you get straight out of ROTC. I know active duty have that opportunity but I don't know about the Reserve Component. You cannot be civil Affairs in the Army National Guard. The more diversity you have and civilians skills you have to offer, the better your Civil Affairs experience will be. I used to be a claims adjuster handling Automotive claims for insurance companies and that skill said helped me with paperwork and keeping track of projects as a Commander's Emergency Response Program Officer while serving in Afghanistan in 2008- 2009. Civil Affairs is great because you can have Personnel from the medical service Corps join civil affairs. Sometimes, you may be working alongside with veterinarians, doctors and nurses. Civil Affairs is a very Diversified Branch or functional area. Being in Civil Affairs does help with your promotions. The selection rate for promotion of officers in Civil Affairs is very high.

As a CA NCO, the top officers I have worked with have been engineers, Intel, and armor (cav, not tanker). Though the beauty of needing to branch something else first allows for a broad spectrum of backgrounds in the job