That sounds a bit vague. Could you give a bit more details about this supposed post World War 2 doctrine?

I was reading somewhere about post-WW2 US foreign policy where the military was supposed to be used not to dominate, but to protect borders enabling peace and trade.

I think its worth mentioning that the US wasn't always overwhelmingly dominant post WW2, militarily. Remember, the US was locked in a competition with the Soviet Union, with both sides trying to get a military edge over one another throughout the Cold War.

And it's not like the US dominating militarily means that the US hasn't been involved in protecting trade or being involved in peacekeeping missions protecting borders. The US led the United Nations Command sent in to Korea to repel the North Koran invasion, and the US Navy has been involved in protecting sea routes around the world for quite some time. In fact, the Naval Act of 1794 authorizing the creation of the original six frigates of the US Navy was an acknowledgement by the US that it needed naval forces to secure its own trade routes and interests overseas. Source: Six Frigates by Ian W. Toll

In terms of foreign policy (to include military policy), you can gleam a lot from the National Security Strategy of the United States, written by successive presidential administrations. An archive of this can be found online here, stretching back to the Reagan era.

I also want to emphasize that the actual post-WW2 military doctrine of the United States had already been envisioned by many strategic thinkers before WW2 had even ended.

In General George C. Marshall's Biennial Reports as Chief of Staff of the Army, completed before Japan had even formally surrendered, Marshall writes extensively about the state of the US military before the war, the challenges it took to organize the massive and unprecedented mobilization of manpower and industry, and what the US had to do in the post-war world.

In his chapter For the Common Defense, starting on page 208, Marshall ponders what the US military must become to prevent a future WW2 and how to be better prepared. He acknowledges that if it weren't for the vast oceans, as well as British and Soviet blood, the US would have suffered considerably more:

In order to establish an international system for preventing wars, peace-loving peoples of the world are demonstrating an eagerness to send their representatives to such conferences as those at Dumbarton Oaks and San Francisco with the fervent hope that they may find a practical solution. Yet, until it is proved that such a solution has been found to prevent wars, a rich nation which lays down its arms as we have done after every war in our history, will court disaster. The existence of the complex and fearful instruments of destruction now available make this a simple truth which is, in my opinion, undebatable.

...

The German armies swept over Europe at the very moment we sought to avoid war by assuring ourselves that there could be no war. The security of the United States of America was saved by sea distances, by Allies, and by the errors of a prepared enemy. For probably the last time in the history of warfare those ocean distances were a vital factor in our defense. We may elect again to depend on others and the whim and error of potential enemies, but if we do we will be carrying the treasure and freedom of this great Nation in a paper bag.

He then also writes:

Twice in recent history the factories and farms and people of me United States have foiled aggressor nations; conspirators against the peace would not give us a third opportunity.

He's quite blunt too about his assessment of the necessity of the US to have forces worldwide:

It no longer appears practical to continue what we once conceived as hemispheric defense as a satisfactory basis for our security. We are now concerned with the peace of the entire world. And the peace can only be maintained by the strong.

And:

The Regular Army must be comprised largely of a strategic force , heavy in air power, partially deployed in the Pacific and the Caribbean ready to protect the Nation against a sudden hostile thrust and immediately available for emergency action wherever required. It is obvious that another war would start with a lightning attack to take us unaware. The pace of the attack would be at supersonic speeds of rocket weapons closely followed by a striking force which would seek to exploit the initial and critical advantage. We must be suffciently prepared against such a threat to hold the enemy at a distance until we can rapidly mobilize our strength. The Regular Army, and the National Guard, must be prepared to meet such a crisis.

It's hard to argue with much of what Marshall has written here with how the US has employed its military post WW2: one that is heavy in airpower, forward deployed across the Pacific and Atlantic oceans, and one that is strong.