It's the end, the war has been lost - or not, it made no difference. On paper, the Battle of Hogwarts had been a victory. Voldemort dead, his forces broken and scattered. But the Order had paid a steep price, the Weasleys all dead, half the DA died manning the walls. The Hogsmeade volunteers dead to a man, Kingsley perished - the victory was hollow. There was no one left to make sweeping changes in the Ministry, no manpower to overthrow the machinery Voldemort built. There were a few trials, kangaroo courts so that others could shift the blame.
This climate led to an unlikely Alliance. Harry Potter, wounded but with unfinished business. Fleur Delacour, with the intent to build a better future for her daughter. And Andromeda Tonks with the thirst for vengeance. The Ministry might be unwilling to deliver justice, but the trio knows who is guilty.