Where is Khurasan (country) situated? Is this Pakistan? There is a Hadith about army of Khurasan bearing the flag when Imam Mahdi will come to the world.

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

According to Mu'jam al-Buldan of Yaqoot al-Hamwi, Khurasan is a vast land that comprises Naisaboor, Haraa, Maru, Balkh, Talqaan, Nasa, Aburd, Sarkhas and all other cities and town before the river Bactrus (Jihon).
Ibn Qutaibah said: 'The people of Khurasan are the people of Da'wah and helpers of the state of Islam' .
They will continue to be the strongest among kings of non-Arabs (Persians …) and do not pay any tribute or royalty.
Khurasan is mentioned in many Hadith.
al-Hakim in his book al-Mustadrak reported from Thauban (Radiya Allahu Anhu) who said that Allah's Messenger (Sallallahu Alaihi wa Sallam) said: "There will be a war between three persons about your treasure, all of them are sons of rulers; none of them will hold authority. Then, the bearers of black flags will come out from the east, who will fight you fiercely. He added saying: If you see him, you have to pay homage to him even if you creep on snow since he is the Khalifa (vice regent) of Allah, al-Mahdi (the well-guided one)" . al-Hakim said: 'This Hadith is sound' .
Another narration in al-Mustadrak and al-Musnad from Thauban (Radiya Allahu Anhu) who said that Allah's Messenger (Sallallahu Alaihi wa Sallam) said: "If you see the black flags coming out from the direction of Khurasan, you have to report this, even creeping, since al-Mahdi Khalifah of Allah will be among them" [ al-Hakim said: 'This Hadith is sound according to Bukhari and Muslim conditions and terms' .
A third Hadith recorded by al-Tirmizi , Ibn Majah , Ahmad , al-Hakim and Abu Ya'lah reads: "al-Dajjal (Anti-Christ) will come out from a land called Khurasan and he will be followed by some nations whose people's faces are like a shield due to being round, large and thick, because of the several layers of leather that cover shield.
Allah knows best.