Why do you think it took from 1906 to 1972 for the U.S. Government to acknowledged that the black units involved in the Brownsville Affair had been...

The answer, pure and simple, is that the U.S. Government
was unwilling to admit that one of its presidents had made a mistake on purely racial
grounds. Although this might have been more likely in earlier days, the fact that it
happened in the twentieth century at a time when racial bias was rampant acted as a
barrier to righting the wrong that had been done.

The black
soldiers involved were obviously wrongfully accused; the evidence of that fact now seems
overwhelming. However, President Theodore Roosevelt, likely fearful of losing the white
Southern vote, had most of the soldiers dismissed not for participating in the violence;
but for disavowing any knowledge of it, which he deemed a "conspiracy of
silence."

The matter was in fact investigated by a number
of historians who believed that a number of the soldiers wrongfully discharged had won
the Congressional Medal of Honor. This proved not to be true, and was an embarrassment
for the historical community. Later, in 1970, another historian, John D. Weaver
published The Brownsville Raid in which he argued that the men were
innocent. His book led to a new investigation by the army which exonerated the men
accused. Even so, the racial climate up until the 1970's was such that any action on the
Brownsville affair was not likely.