is it ever acceptable for the US government to knowingly violate the constitution?

By the time Wilson reached Paris in December 1918, political liberty had been snuffed out in America. "One by one the right of freedom of speech, the right of assembly, the right to petition, the right to protection against unreasonable searches and seizures, the right against arbitrary arrest, the right to fair trial . . . the principle that guilt is personal, the principle that punishment should bear some proportion to the offense, had been sacrificed and ignored." So an eminent Harvard professor of law, Zechariah Chafee, reported in 1920. The war served merely as pretext. Of that there can be little doubt. In a searing civil conflict that threatened the very survival of the republic, Americans, under Lincoln, enjoyed every liberty that could possibly be spared. In a war safely fought three thousand miles from our shores, Americans, under Wilson, lost every liberty they could possibly be deprived of.

haha i can hear every libertarian in america jumping on those last couple sentences, since the revisionist line on lincoln is that he was a ruthless dictator who ran roughshod over the constitution - suspended habeas corpus, imprisoned newspaper editors, et al - to achieve his sole goal of oppressing the southern states (slavery not being an issue, as revisionist-lincoln didn't care about it and revisionist-civil war didn't have anything to do with it). but i can't imagine anyone but the most steadfast states'-rights extremist arguing that lincoln's actions hurt the constitution - it wasn't an unnecessary war like iraq (or WWI), it was, as karp says, something "that threatened the very survival of the republic." and if anything the first successful years of reconstruction led to a far stronger and more inclusive nation than we'd ever had before (or would have, if reconstruction's end in 1876 didn't sabotage a lot of what had been done). but other than that, i can't think of many times where a president violated the constitution for anything other than purely nasty, self-serving ends.