What is narrated in the Sunnah about the invasion of India
Question

                I have a question about the invasion of India, and the claims made by some people that it will happen. They say that the one who is killed during that invasion will enter Paradise, and they quote as evidence this hadith: It was narrated by an-Nasaa’i (hadith no. 3124) that Thawbaan, the freed slave of the Messenger of Allah (blessings and peace of Allah be upon him), said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “There are two groups of my ummah whom Allah will protect from the Fire: the group that invades India, and the group that will be with ‘Eesaa ibn Maryam (peace be upon him).”

In Sunan an-Nasaa’i (hadith no. 3123) it is also narrated that Abu Hurayrah said: The Messenger of Allah (blessings and peace of Allah be upon him) promised us that we would invade India. If I live to see that, I will offer myself and my wealth in that campaign. If I am killed, I will be one of the best of the martyrs, and if I come back I will be Abu Hurayrah the liberated (from Hellfire).” 

First of all, are these hadiths saheeh? If they are, then do they really refer to a war in the subcontinent towards the end of the world?
Praise be to Allah.
      Firstly: 

    With regard to the first hadith, which is the hadith of 
    Thawbaan, the freed slave of the Messenger of Allah (blessings and peace of 
    Allah be upon him), it is a saheeh hadith. 

    It was narrated by an-Nasaa’i in his Sunan (3175) and 
    classed as saheeh by al-Albaani in Saheeh an-Nasaa’i. It was also 
    narrated by Ahmad in his Musnad (37/81), and classed as hasan by the 
    commentators. 

    With regard to the second hadith, which is the hadith of Abu 
    Hurayrah, it is a da‘eef (weak) hadith. 

    It was narrated by an-Nasaa’i (3173) and classed as da‘eef by 
    al-Albaani in Da‘eef an-Nasaa’i. it was also narrated by Ahmad in his
    Musnad (12/28 – ar-Risaalah edition), and classed as da‘eef by the 
    commentators. There are other isnaads for the hadith of Abu Hurayrah, all of 
    which are da‘eef, and there are other hadiths on this topic that are not 
    free of problems. What has been proven and established to be saheeh is the 
    hadith of Thawbaan. 

    Secondly: 

    India was indeed invaded and taken over by the Muslims, and 
    they destroyed its idols, on several occasions. That began at the time of 
    al-Waleed ibn ‘Abd al-Malik, under the leadership of al-Qaasim ibn Muhammad 
    ath-Thaqafi. It was also invaded and taken over at the time of the 
    ‘Abbasids, under the leadership of the commander Mahmoud ibn 
    Sabuktigin 
    (may Allah have mercy on him). He took it upon himself to invade India once 
    every year. Shaykh Siddeeq Hasan Khan (d. 1307 AH – may Allah have mercy on 
    him) said: 

    With regard to India, it was conquered during the reign of 
    al-Waleed ibn ‘Abd al-Malik, at the hands of Muhammad ibn Qaasim ath-Thaqafi 
    in 92 AH, and his banners reached from the borders of Sindh to Kannauj in 95 
    AH. After he returned, the rulers of India returned to their places and the 
    rulers among the Marwanid and ‘Abbasid caliphs remained in Sindh. … The 
    author of al-Mughni said: … At the end of the fourth century  AH, 
    Sultan Mahmoud al-Ghaznawi set out to invade India, and he came several 
    times; he was victorious and he captured war booty, and he wrested Sindh 
    from the governors of the ‘Abbasid caliph al-Qaadir Billah ibn al-Muqtadir. 
    But Sultan Mahmoud did not stay in India. His sons controlled a region 
    stretching from Ghazneen to Lahore until the Sultan Mu‘izz ad-Deen Saam al-Ghuri 
    took over Ghazneen, and came to Lahore and captured Khusrau Malik, who was 
    the last ruler the Ghaznawi dynasty. He seized control of India and made 
    Delhi the seat of government in 589 AH. From that date until the end of the 
    twelfth century, the kingdoms of India were in the hands of Muslim Sultans.
    

    Abjad al-‘Uloom 
    (1/344-345). 

    Ibn Katheer (may Allah have mercy on him) said concerning the 
    events of the year 94 AH: In that year, al-Qaasim ibn Muhammad ath-Thaqafi 
    conquered India and captured booty that cannot be enumerated or described.
    Al-Bidaayah wa’n-Nihaayah (9/113) 

    Ibn Katheer (may Allah have mercy on him) said concerning the 
    events of 96 AH: In that year, Yameen ad-Dawlah – Mahmoud ibn 
    Sabuktigin – invaded India, conquering 
    major cities and seizing a large amount of booty. He took captive one of 
    their kings – namely the king of Karachi – who fled when he conquered it, 
    and he broke their idols.

    Al-Bidaayah wa’n-Nihaayah 
    (11/358). 

    He also (may Allah have mercy on him) said concerning the 
    events of 149 AH: in that year, Mahmoud ibn Sabuktigin invaded India, and 
    met the king of India in battle. There was a great deal of fighting among 
    the people, then the fighting ended with a great defeat for India, and the 
    Muslims killed many of them and captured a great deal of booty from them, 
    including gemstones, gold and silver. They also took from them two hundred 
    elephants and they tried to pursue those who fled the battlefield, and 
    destroyed many temples. Al-Bidaayah wa’n-Nihaayah (12/8) 

    What happened of the invasion of India is one of the signs of 
    Prophethood, because the Prophet (blessings and peace of Allah be upon him) 
    foretold its happening. This was knowledge of the unseen that Allah revealed 
    to him. Ustadh Muhammad ‘Abd al-Wahhaab Buhayri (may Allah have mercy on 
    him) said: It should be noted: the hadith mentioned – i.e., the hadith of 
    Thawbaan – is one of the signs of Prophethood. The Muslims invaded India and 
    Sindh at the time of the Umayyads – then he mentioned some incidents that 
    occurred during the invasion and conquest of India. Buloogh al-Amaani min 
    Asraar al-Fath ar-Rabbaani (22/411) 

    There is nothing to indicate that there will be another 
    invasion of India that will occur at the end of time, close to the onset of 
    the Hour or during the time of ‘Eesaa (peace be upon him), as was suggested 
    by some scholars. Rather what appears to be the case is that what is 
    referred to in the hadith is what actually happened. The hadith of Thawbaan 
    does not mention the connection between the two groups; rather each group 
    has its own time, although both of them may share the same virtue. 

    And Allah knows best.