What was the purpose of William B. Travis' letter from the Alamo?

To ask for aid and to state the determination of the defenders to fight to the death.

Travis knew he was in a bad state. His small army of 150 or so were surrounded by three to four thousand Mexicans under the command of Santa Anna.

He needed help. He pleaded with anyone to "come to our aid with all dispatch"--that is, as quickly as possible.

But he also felt the need to state that he and his men were prepared to die if need be. "We will never surrender," he stated. And he ended his letter, "Victory or Death."

He and his men got the latter after a brief battle.