Custer lost for several reasons.
Tribes set aside their differences and joined together amassing numbers in the thousands to include Cheyenne, Sioux, Arapaho and others. Custer, ignored orders and gave orders to attack, not realizing the lands he would have to cover to attack and not understanding the sheer number of Indians.
He was outnumbered, outfought, out thought and arrogant in his beliefs that he could easily kill "the savages". If he had lived, he most likely would have been court martialed for disobeying direct orders.
Alternate version Custer split his command in 3- sending Benteen\'s detachment South to ensure the native Americans did not escape up-river, Reno\'s detachment was to ford the river, and set up a skirmish line outside the Village to its South. This was intended to draw the warriors out to set up a defensive line South of the Village. Custer rode North on the near side of the river to lead a charge from the north end of the camp, once most of the warriors had already moved south to engage Reno. This tactic had worked well in the past...

As it forced the irregular Indians to form a defensive line to cover their women and children, and allowed Custer to attack their rear and take possession of their camp, which was essentially everything they had. Reno was beat down to a defensive position back across the river where Benteen eventually reinforced him and they held out for three days. Custer\'s detachment was killed to a man. In the days after Custer\'s defeat an army column led by General Terry arrived and went over the evidence on the scene. They found that Custer had ridden North across the river from the encampment until he got about half way, and then he had made for the river. The tracks left by his column showed that the column entered the river, but never made it to the other side. Rather, the tracks showed them entering the river in good order, but leaving in disarray, ultimately ending up at the site of the massacre. That portion of the Village was the Cheyenne encampment. The Cheyenne oral tradition tells the story of the four warriors. When the women saw the cavalry column coming down the far embankment of the river, they raised the alarm, but most of the warriors had already ridden South toward the sound of Reno\'s detachment hotly engaged. There were only 4 warriors within earshot to defend the center of the camp against Custer\'s 200 men. As the cavalry was in mid river, the four Cheyenne charged and got off only a single volley. One man fell from his saddle into the river, and the Cheyenne report was that the column stopped dead, picked up the fallen man, and retreated.

"Portrait of Maj. Gen. (as of Apr. 15, 1865) George A. Custer, Officer of the Federal Army." Between 1860 and 1865. Selected Civil War photographs, 1861-1865, American Memory collections, Library of Congress.
AUDIO CREDIT: Warde Ford, unaccompanied vocals. "Custer\'s Last Charge." Recorded by Sidney Robertson Cowell. December 26, 1938. California Gold: Northern California Folk Music from the Thirties, Library of Congress.