system
Given the following post, respond as if you're commenting on Reddit:
human
[LOTR] Why didn't Gondor occupy Mordor after defeating Sauron after the War of the Last Alliance?

Wouldn't doing so allow early warning of Sauron's return and prevent Sauron from rebuilding his armies?
gpt
They did. Gondor occupied Mordor for 2000 years until the Great plague and the Kin Strife (Gondorian civil war) weakened the Kingdom so much that the fortresses couldn’t be maintained. 

Basically Gondor built the Towers of Teeth at the Black Gate, Cirith Ungol, Durthang, and several other fortresses and roads that Sauron later used.