I feel aprehensive about writing this answer as I'm not really gonna cite any sources. So if inspite of that you'll find this answer useful, great!

I had a history of journalism class, and a thing we learned about the Vietnam War was that it was the war which allowed for the most free war press coverage in history.

Journalists had complete autonomy, being able to record, describe and quickly share the actual reality of war. Which Americans saw for the first time on their TV screens, during their evening news report.

Now, up until then, war was usually covered by journalists who were kept at the military bases, under the watchful eye of the generals, who essentially dictated "the news", which were then sent back to the press. The best example of this was the 1st World War, where press was turned into the biggest propaganda machine, in the history of mass communications, up until then.

The media influence on the people's reaction to the Vietnam War, lead to a rethinking about freedom of the press, atleast in regards to the coverage of war.

That is when the so called "pool" appeared, which became very similar to what was happening in WWI - war media journalists were being estranged from the actual battlefield, where the reality to be reported on existed, and were kept, once more, close and under the watchful eye of government bodies.

In regards to this, there seems to be an interesting article online: https://www.pbs.org/weta/reportingamericaatwar/reporters/amanpour/poolsystem.html

I know the text is in a rather informal manner (no citations or authors) - hopefully that doesn't bother you too much.

Well, hope i added some relevant information in regards to the question.