@oristarA,
From 1608 onward, the Iroquois Confederation had been the inveterate enemies of the French. I won't go into the causes here, it's not germane. But that made them the allies of the British on the basis of "the enemy of my enemy is my friend." During the French and Indian War (1753-1760--the dates are often given as 1754--1763, but the fighting was effectively over by 1760), the Iroquois Confederation was a staunch ally of the British, although their performance was lacklusert--but they were fighting the old enemy, the French. They were lead by William Johnson, the British agent for Indian Affairs. Johnson died before the revolution, but he had already secured the loyalty of the Iroquois in his own lifetime (and made himself quite wealthy in the process) He was, by custom of the Iroquois, a member of the nations because he had married a Mohawk woman. When the revolution broke out, the Iroquois honored their old alliance, and largely because of the deep respect they held for Johnson (they really did not understand how much land they had lost to him when they signed treaties which they honored because of the Johnson legacy). For many, many years while Johnson was the Indian Affairs agent, whites and Indians had lived peacefully, and a great many white settlers had moved into the Mohawk Valley.
When the revolution broke out, those white settlers were now in jeopardy, and there were many atrocities committed by Indian allies of the British, not all of them Iroquois--but the Iroquois nations were blamed. Early in the war, the British had concocted a plan to drive a wedge between the New England states and the mid-Atlantic states. It was a sound plan, but it was not executed as it ought to have been. British command was divided, and many of the general officers were inept. One column was to march sought under Burgoyne, Clinton was to have marched north from New York, and Lt. Colonel Barry St. Leger was to march east along the Mohawk Valley. Clinton not only did not march north, he set off on a hare-brained expedition to take Philadelphia, where the Second Continental Congress sat, and so end the war in one blow. It was a silly idea, and although it produced much bloodshed, it not only failed, it doomed Burgoyne's expedition.
St. Leger did march east to the Mohawk River, and his first move was to take Fort Stanwix. He didn't have the troops, however, to take it by assault, so he laid siege to it. General Herkimer, leading a force of somewhat fewer than a thousand white militia and Oneida Indians (the Oneida were one of the Iroquois-speaking tribes, but they had remained loyal to their white neighbors, who were largely pro-revolution).
At the Battle of Oriskany (clickity-click!), Herkimer encountered a force lead by John Johnson, William Johnson's son, and consisting of Indians loyal to Britain. It was a small, but very bloody affair. White militiamen captured in the battle were tortured, mutilated and murdered by the Indians. In fact, the Indians were so caught up in torture and scalping, that the Americans were able to counterattack. They lost, but they managed to get away. The commander, General Herkimer, was fatally wounded, so the small force lacked leadership, and retreated.
All of that took place in August, 1777. Benedict Arnold was leading a relief force to lift the siege of Ft. Stanwix, and rather cleverly started a rumor that he was coming with a large force. St. Leger became dismayed, sending a report that Arnold was coming with 3000 Continentals (i.e., regular troops--it was total hogwash). The Indian allies were disenchanted with siege warfare, and the lack of plunder, and they began to desert. St. Leger soon abandoned the siege and retreated to Lake Ontario. Arnold marched back to the Hudson River, and in September, with Arnold playing an important role, Burgoyne's army was defeated, surrendered a few days later. It was arguably the most important event in the revolution--a few months later, in 1778, France recognized the United States, and began giving substantial material aid.
For the white settlers in upstate New York, the nightmare was not over. Bands of Indians, many of them not members of the Iroquois Confederation, were plundering farmstead and attacking remote settlements. Washington, who had spent years fighting the Indians in the French and Indian War, decided that something must be done. People can argue all day along about who did what to whom and when--the public perception was that the Iroquois were traitors and heathen devils, and must be exterminated. The survivors of General Sullivan's campaign fled to Canada, where the Mohawk, as they are called (sometimes they are called the Six Nations) reside to this day.