Does that happen to veteran sergeants too who had seen more combat. No offense to anyone but seargeants today had probably seen much less combat then the ww2, Korea and Vietnam generations. Did the same thing happen to seargents who were veterans of bloody and long conflicts like WW2 and Vietnam?
That question is really impossible for me(or anyone) to answer conclusively. But I can toss out a couple nuggets of information that might shed some light on it.
1) Different specific figures get tossed around all the time. But the conventional wisdom is that only around 10-20% of US WW2 soldiers would fire at another human being when it came down to infantry warfare. In Vietnam that number supposedly climbed to 40-50%, and in the modern US Army army it is supposedly around 75%. The changes are largely due to changes in training(man-shaped pop-up targets for rifle qualification etc), and the fact that we have moved to an all volunteer Army. The guys volunteering for Combat Arms, in theory, should have no mental or emotional issues with killing other people.
2) Back during WW2, US Army sergeants were promoted by their units. Nowadays there is a centralized promotion board. What this means is that sergeants today play the game, get their promotion points, their correspondence courses, sundry other things, and get promoted according to a fairly set time-table. Sergeants in the old days were promoted because of merit(or favoritism, take your pick), meaning that those soldiers who demonstrated courage and leadership under fire could immediately get promoted by their respective Battalion or Regimental officers. Which of course would more rapidly allow the US to field a battlehardened corps of NCOs.
3) But yes. US(heck all armies I'm sure) soldiers, from the lowest private, to grizzled sergeants, to officers of all ranks have frozen up in all our wars. It isn't recorded or really mentioned in the academic historical books, or journalism of the periods, but read the personal accounts and biographies of combat soldiers and you'll see exactly what I am talking about.
In Audie Murphy's To Hell and Back
for example, there are several cases of soldiers of various ranks freezing up in the midst of combat.