expression, ichidan verb
hoist a flag; raise a flag
Literal action of raising a flag on a pole or similar. Can also be used figuratively for setting a goal or making a declaration.
開会式で旗を立てる。
They raise the flag at the opening ceremony.
彼は新しいプロジェクトの旗を立てた。
He raised the flag for a new project (i.e., launched it).
旗を揚げる also means to raise a flag, but 揚げる emphasizes hoisting something upward, while 立てる focuses on setting it upright. 旗を立てる is more common for the literal action.
Compound of 旗 (flag) + を (object marker) + 立てる (to stand/raise). The expression is transparent from its components.