If they gain dominance over you, they would be [i.e., behave] to you as enemies and extend against you their hands and their tongues with evil, and they wish you would disbelieve.
In the Name of Allah, the Most Gracious, the Most Merciful.
Reason behind revealing Surat Al-Mumtahanah
The story of Hatib bin Abi Balta`ah is the reason behind revealing the beginning of this honorable Surah. Hatib was among the Early Emigrants and participated in the battle of Badr. Hatib had children and wealth in Makkah, but he was not from the tribe of Quraysh. Rather, he was an ally of `Uthman. When the Messenger of Allah ﷺ decided to conquer Makkah, after its people broke the peace treaty between them, he ordered the Muslims to mobilize their forces to attack Makkah, and then said,
إِن يَثْقَفُوكُمْ يَكُونُوا لَكُمْ أَعْدَاءً وَيَبْسُطُوا إِلَيْكُمْ أَيْدِيَهُمْ وَأَلْسِنَتَهُم بِالسُّوءِ (Should they have access to you, they will become your enemies, and will stretch their hands and tongues towards you with evil;...60:2). The verse indicates how bitter are the feelings of disbelievers towards Muslims. They would use all means, their hands and their tongues, and would spare no effort to harm the Muslims. Therefore, it is impossible that when they find an opportunity in their favor and overpower Muslims, they will ever exercise tolerance towards them.
وَوَدُّوا لَوْ تَكْفُرُونَ (...and they desire that you should reject the (true) faith...60:2). The greatest desire of the non-believers' hearts is to see the Muslims revert to disbelief. Thus when the Muslims stretch their hand of friendship towards them, it will be at the cost of their belief. They will never be happy with the Muslims unless they give up their faith.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 2–4
During the state of war, when the position is that Islam and non-Islam have become polarised on separate fronts, it is the responsibility of the people of Islam to sever their relations with the non-Islamic front, even if their relatives and dear ones are part of it. Being a believer in Truth and at the same time having close relations with the enemies with whom one is at war is totally wrong.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.