What does the green beret signify in the movie 'the deer hunter'?

i know that the 3 of them were going out out war..but, what does the green beret signify? and when he said, ';**** it'?What does the green beret signify in the movie 'the deer hunter'?
In the 1960s, the US Army was developing something called ';Special Forces'; that were designed to help native troops fight against Communist forces from The USSR, 'Red' China, North Vietnam, etc. John F. Kennedy liked these elite forces and their small political imprint and plausible deniability, and he liked their head gear, too, the green beret.





Soon after, a sergeant recorded a song where he identified the soldiers as ';Green Berets'; but, as anyone who has earned one might tell you, they are ';Special Forces Troopers'; while a green beret is a hat!





Robert DeNiro's character (at least) became a Special Forces soldier and wore the beret.