I wouldn't say they were sucked into any ambush, Peace. Rather, they arrived unexpectedly as unwanted "guests" at the biggest Indian village ever seen in that region and were refused entrance (to put it mildly), swarmed upon, and annihilated...but not without putting up a courageous fight.

If by "ambush" you mean the few Indians who stopped Custer from making the river crossing...well, that's one of many conflicting stories from many different eyewitnesses of the action, and I don't think anyone will ever know for sure exactly what happened.

One story suggests that Custer himself was seriously injured by a rifle shot from those few warriors just at the beginning of the fight, and that this injury aborted his unit's attempt to cross the river.

Other stories suggest that in uninjured Custer led some of his men in a retreat to "Last Stand Hill" while other small groups of soldiers got scattered in various different directions. The group on the hill attempted to make a defensive position behind their dead horses and engaged in volley fire for some time but were eventually overwhelmed and all killed, as were all the other troopers and scouts in Custer's immediate forces (as opposed to those others under the command of Reno and Benteen).