What is the ONE thing EVERY young Officer must know?

My advice, like that of many other officers and senior NCOs, is develop a mentoring relationship with one or more of YOUR senior NCOs. While they must know (and good ones will expect this) that you are the final decision-maker and that you are looking for their perspective on issues, you can avoid a lot of (most) discipline problems by working with and through your senior NCOs. Having said this, you need to also be careful of suck-ups and manipulators.

Listen to you senior NCO/Chief

One simple rule: LEAD WITH YOUR EARS!!!

Through tough experience, the most important thing I learned as a young 2LT was to listen - to everyone at all times.

If logistics isn't your main branch, learn it with all diligence!! Know the supply and maintenance systems like the back of your hand. If you can get a good grasp and understanding about beans and bullets, your Soldiers will LOVE you to death even if you don't know exactly what they do since they're the subject matter experts. So get knee deep with the supply and maintenance programs and everything else will eventually fall into place and you'll be seen as the LT that gets crap done and your Soldiers will appreciate that immensely since getting critical resources will allow your platoon to outshine others. Plus, your Platoon Sergeant will appreciate it as well so he/she can spend more time training your Soldiers while you're making things happen behind the scenes. Remember, supply and maintenance.

In the Horse Calvery, an officer did not eat until the horses were bedded down and the Troopers fed. My father, an Army Air Corps ground pounder in the Pacific during WW II, told me this with the admonition: take care of your equipment and your troops and they will take care of you. I followed his advice, adding "report up problems with either equipment or troops and get them solved ASAP.

1. Expect to learn
2. Expect to fail (I would almost encourage it...no better way to learn)
3. Remain professional, keep a sense of humor, do the things you don't want to do first; your commander is watching, and so are your troops.
4. Take the blame for your people...and also give them the credit for getting the job done.
5. Do the job with the mindset of wanting to contribute, and in the best interest of the mission
6. You will start to find success...and you will be a leader.

Something less obvious and possibly not stated (there were way too many to read them all) would be effective writing skills. In today's email-centric environment where you may never meet half of the people you communicate with, you are what you write.

probably that there's a lot more to know than "one thing."

Put in extra hours to learn what your platoon sergeant knows.