He wanted to unite all native tribes against white people
Tecumseh was a precusor of Panindianism since he wanted to unite all native tribes to fight agianst white rule.
Why did Tecumseh want to unite Indian nations?
Tecumseh wanted to unite all the tribes so that the American government would have to deal with them as a unit not individually.
Tecumseh goal was to stop American settlement of the Ohio River Valley as had been promised by the British in 1763. Many of the tribes in Ohio, Kentucky and Indiana joined him.
His followers had gathered a large army at prophet's town under the command of Tecumseh younger brother. This army and the political efforts of Tecumseh were seen as a threat to the American settlers who want the land of the Ohio Valley. In 1811 William Harrison commanded a force that destroyed Tecumseh army at Prophet's town destroying the threat.
Tecumseh did not give up his hope of an independent united nation of native tribes. He gathered an army to fight with the British in the war of 1812 until his death in a battle in 1813.
The Americans were upset because if Tecumseh dream had come true the Ohio River Valley, would have become closed to American settlers and the expansion of the United States.