The Surah derives its name from verse 71 and 73 in which the word zumar has occurred.
In verse 10 (wa ardullah-i-wasi atun : and Allah's earth is vast) there is abundant evidence that this Surah was sent down before the migration to Habash. Some traditions provide the explanation that this verse was sent down in respect of Hadrat Ja'far bin Abi Talib and his companions when they made up their mind to emigrate to Habash.(Ruh al-Maani, vol. XXII, p. 226).
The entire Surah is a most eloquent and effective address which was given some time before the emigration to Habash, in an environment filled with tyranny and persecution, ill-will and antagonism, at Makkah. It is a sermon whose addressees mainly are the unbelieving Quraish, although here and there the believers also have been addressed. In it the real aim of the invitation of Muhammad (upon whom be Allah's peace and blessings) had been enunciated, which is this:Man should adopt Allah's servitude sincerely, and should not pollute his God worship with the service of any other. Presenting this cardinal principle in different ways over and over again, the truth of Tauhid and the excellent results of accepting it, and the falsehood of shirk and the evil consequences of following it, have been explained in a most forceful way, and the people exhorted to give up their wrong way of life and return to the mercy of their Lord. In this very connection, the believers have been instructed, as if to say:"If a place has become narrow for the worship and service of Allah, His earth is vast: you may emigrate to some other place in order to save your faith: Allah will reward you for your patience."On the other hand, the Holy Prophet has been encouraged, so as to say:"Tell the disbelievers plainly that they may do whatever they like, but their persecutions and tyrannies will never deter you from the way of Islam; that they may go on doing their worst to obstruct your way, but you will continue to perform your mission in spite of the adverse conditions and circumstances."
Source: Sayyid Abul Ala Maududi - Tafhim al-Qur'an - The Meaning of the Quran