There are signs that precede the reappearance. You can see them here:
Signs of Reappearance – Sayed Mohammad Baqer Qazwini
And then there are signs of the Day of Judgment:
Signs of the Hour – Sayed Mohammad Baqer Qazwini
So the the massive events that will take place on earth (collision of stars, destruction of mountains…) will be tied to the Day of Judgment, which is after the reappearance of Imam Mahdi (a).
When the Zahoor occurs, there will be some wars here and there. Some evil groups will try to fight Imam Mahdi (a). How big those wars will be is not clear to us.