I have heard from a friend of mine that Islam directs all the Muslims living in non-Muslim lands to unite under one leadership and present themselves as a single entity. Is this true?
I am afraid that nowhere has Islam directed Muslims living in a non-Muslim country to unite under one leadership. This may serve their interest and be very beneficial for them. However, they have not been bound by their religion in this regard. It is up to them if they want to adopt such a policy.
Some people do present the following verse to contend that Islam has directed Muslims to politically unite:
إِنَّ هَذِهِ أُمَّتُكُمْ أُمَّةً وَاحِدَةً وَأَنَا رَبُّكُمْ فَاعْبُدُونِي (٢١: ٩٣)
Indeed, this Ummah of yours is a single Ummah, and I am your Lord and Cherisher. (21:93)
If the context of this verse is deliberated upon, it comes to light that the Qur’ān is not directing the present Muslim Ummah to remain united; on the contrary the word Ummah here is used for all the Prophets which are mentioned in the preceding verses (78-91). After enlisting most Prophets, the Qur’ān says that all these Prophets are one Ummah in the sense that they brought the same religion and it is the people who introduced innovations in it:
وَدَاوُودَ وَسُلَيْمَانَ إِذْ يَحْكُمَانِ فِي الْحَرْثِ إِذْ نَفَشَتْ فِيهِ غَنَمُ الْقَوْمِ وَكُنَّا لِحُكْمِهِمْ شَاهِدِينَ فَفَهَّمْنَاهَا سُلَيْمَانَ وَكُلًّا آتَيْنَا حُكْمًا وَعِلْمًا وَسَخَّرْنَا مَعَ دَاوُودَ الْجِبَالَ يُسَبِّحْنَ وَالطَّيْرَ وَكُنَّا فَاعِلِينَ وَعَلَّمْنَاهُ صَنْعَةَ لَبُوسٍ لَكُمْ لِتُحْصِنَكُمْ مِنْ بَأْسِكُمْ فَهَلْ أَنْتُمْ شَاكِرُونَ وَلِسُلَيْمَانَ الرِّيحَ عَاصِفَةً تَجْرِي بِأَمْرِهِ إِلَى الْأَرْضِ الَّتِي بَارَكْنَا فِيهَا وَكُنَّا بِكُلِّ شَيْءٍ عَالِمِينَ وَمِنْ الشَّيَاطِينِ مَنْ يَغُوصُونَ لَهُ وَيَعْمَلُونَ عَمَلًا دُونَ ذَلِكَ وَكُنَّا لَهُمْ حَافِظِينَ وَأَيُّوبَ إِذْ نَادَى رَبَّهُ أَنِّي مَسَّنِي الضُّرُّ وَأَنْتَ أَرْحَمُ الرَّاحِمِينَ فَاسْتَجَبْنَا لَهُ فَكَشَفْنَا مَا بِهِ مِنْ ضُرٍّ وَآتَيْنَاهُ أَهْلَهُ وَمِثْلَهُمْ مَعَهُمْ رَحْمَةً مِنْ عِنْدِنَا وَذِكْرَى لِلْعَابِدِينَ وَإِسْمَاعِيلَ وَإِدْرِيسَ وَذَا الْكِفْلِ كُلٌّ مِنْ الصَّابِرِينَ وَأَدْخَلْنَاهُمْ فِي رَحْمَتِنَا إِنَّهُمْ مِنْ الصَّالِحِينَ وَذَا النُّونِ إِذْ ذَهَبَ مُغَاضِبًا فَظَنَّ أَنْ لَنْ نَقْدِرَ عَلَيْهِ فَنَادَى فِي الظُّلُمَاتِ أَنْ لَا إِلَهَ إِلَّا أَنْتَ سُبْحَانَكَ إِنِّي كُنتُ مِنْ الظَّالِمِينَ فَاسْتَجَبْنَا لَهُ وَنَجَّيْنَاهُ مِنْ الْغَمِّ وَكَذَلِكَ نُنْجِي الْمُؤْمِنِينَ وَزَكَرِيَّا إِذْ نَادَى رَبَّهُ رَبِّ لَا تَذَرْنِي فَرْدًا وَأَنْتَ خَيْرُ الْوَارِثِينَ فَاسْتَجَبْنَا لَهُ وَوَهَبْنَا لَهُ يَحْيَى وَأَصْلَحْنَا لَهُ زَوْجَهُ إِنَّهُمْ كَانُوا يُسَارِعُونَ فِي الْخَيْرَاتِ وَيَدْعُونَنَا رَغَبًا وَرَهَبًا وَكَانُوا لَنَا خَاشِعِينَ وَالَّتِي أَحْصَنَتْ فَرْجَهَا فَنَفَخْنَا فِيهَا مِنْ رُوحِنَا وَجَعَلْنَاهَا وَابْنَهَا آيَةً لِلْعَالَمِينَ إِنَّ هَذِهِ أُمَّتُكُمْ أُمَّةً وَاحِدَةً وَأَنَا رَبُّكُمْ فَاعْبُدُونِي وَتَقَطَّعُوا أَمْرَهُمْ بَيْنَهُمْ كُلٌّ إِلَيْنَا رَاجِعُونَ (٢١ :٧٨-٩٣)
And remember David and Solomon, when they gave judgement in the matter of the field into which the sheep of certain people had strayed by night: we did witness their judgement. To Solomon We inspired the [right] understanding of the matter: to each [of them] we gave judgement and knowledge; it was Our power that made the hills and the birds celebrate Our praises, with David: it was We who did [all these things]. It was We Who taught him the making of metal coats of mail for your benefit to guard you from each other’s violence. Will you then be grateful? [It was our power that made] the violent wind flow [tamely] for Solomon to his order to the land which We had blessed: for We do know all things. And of the evil ones, were some who dived for him, and did other work besides; and it was We who guarded them. And [remember] Job, when he cried to his Lord: ‘Truly distress has seized me, but You are the Most Merciful of those that are Merciful’. So We listened to him: We removed the distress that was on him, and We restored his people to him, and doubled their number, as a Grace from Ourselves, and a thing for commemoration for all who serve Us. And [remember] Ismā‘īl, Idrīs, and Dhu‘l-Kifl, all [men] of constancy and patience. We admitted them to our mercy, for they were of the righteous ones. And remember Dhu‘l-Nūn, when he departed in wrath. He imagined that We would not call him to account! But he cried through the depths of darkness: ‘There is no god but You; glory to You. I was indeed wrong!’ So We listened to Him and delivered him from distress and thus do We deliver those who have faith. And [remember] Zakariyyah, when he cried to his Lord: ‘O my Lord! Leave me not without offspring, though You are the best of inheritors’. So We listened to him and We granted him Yahyā. We cured his wife’s [barrenness] for him. These were ever quick in emulation in good works; they used to call on Us with love and reverence, and humble themselves before Us. And [remember] her who guarded her chastity: we breathed into her of Our Spirit, and We made her and her son a sign for all peoples. Indeed, this Ummah of yours is a single Ummah, and I am your Lord and Cherisher: therefore serve Me [and no other]. But [the later generations] cut off their matter [of unity], one from another: [yet] will they all return to Us. (21:78-93)
In other words, the words ‘Indeed, this Ummah of yours is a single Ummah’ if interpreted keeping in view the context refers to the collectivity of the Prophets that came before Muhammad (sws). They have nothing to do with the Muslim Ummah.