Sanhedrin 98a: If you see many afflictions besetting the generation like a river, wait for Mashi'ach - "Ki Yavo cha'Nahar Tzar... u'Va l'Tziyon Go'el."

Rashi: As long as Tziyon is desolate, the redeemer did not come.

Radak (19): After Ru'ach Hashem will eradicate Gog and Magog, the entire world will fear Him. Even though Yisrael will be in Yerushalayim before this, since Gog will come, they will say that the Go'el did not come, since the Goyim came to fight them. After Hashem will judge him with plague, blood, flooding rain and giant hailstones, they will say that the redeemer truly came. All of Yisrael will fully repent.

Malbim: The redeemer will return the crown to Tziyon, the Mikdash, and the Sanhedrin. He will come also to "those who repent from Pesha in Yakov" - to Bnei ha'Golah, who will repent via the oppressor.