Quran 9:111 – “Allah hath purchased of the believers their persons and their goods; for theirs (in return) is the garden (of Paradise): they fight in His cause, and slay and are slain: a promise binding on Him in truth, through the Law, the Gospel, and the Quran: and who is more faithful to his covenant than Allah? then rejoice in the bargain which ye have concluded: that is the achievement supreme.”
When we read in the Quran, there are limits set by Allah, take for example this passage. Allah says, “Fight in the cause of Allah those who fight you, but do not transgress limits; for Allah loveth not transgressors.” – Quran 2:190
In light of the Quranic passage (2:190) mentioned, it is easily understood that the verse teaches self-defence in a form of a physical jihad (striving in the path of Allah). So, the passage Q. 9:111 endorses self-defence in war. Quran 9:111 cannot be taken to mean to fight the innocent.
Using Quran 2:190 it is evident that to ‘fight in His (God’s) cause’, is meant to fight back in self-defence. So people are free from tyranny and oppression. Moreover, Q 9:111 was revealed because the Byzantine (Roman) Empire mobilised troops in order to attack the Muslim community. As it is reported in three authentic Hadiths.
He (Hadrat ‘Umar further) said: I had a companion from the Ansar and, we used to remain in the company of the Messenger (ﷺ) turn by turn. He remained there for a day while I remained there on the other day, and he brought me the news about the revelation and other (matter), and I brought him (the news) like this. And we discussed that the Ghassanids were shoeing the horses in order to attack us. Id y companion once attended (the Apostle).
And then came to me at night and knocked at my door and called me, and I came out to him, and he said: A matter of great importance has happened. I said: What is that? Have the Ghassanids come? He said: No, but even more serious and more significant than that: the Prophet (ﷺ) has divorced his wives. 
Ibn Sa’d’s Kitab al-Tabaqat al-Kabir
“They (narrators) saud: It (report) reached the Messenger of Allah, may Allah bless him, that the Romans had concentrated large forces in Syria, that Heraclius had disbursed one year’s salary to his soldiers, and that tribes of Lakhm, Judham, ‘Amilah and Ghassan had joined hands with him. They had sent their vanguards to al-Balqa. The Messenger of Allah, may Allah bless him, summoned the people to march.
He set out and informed them about the place which he intended, so that they could make necessary preparations. He sent (messengers) to Makkah and to the tribes of Arabia (asking them) to send help. This took place in the days of intense heat.” 
Kitab Futuh Al-Buldan – al-Imam Abu’l Abbas Ahmad Ibn Jabir Al Baladhuri
“Tabuk make terms. When in the year 9 AH the Prophet marched to Tabuk in Syria for the invasion of those of the Greeks, Amilah, Lakhm, Judham and others whom he learnt had assembled against him, he met no resistance. So he spent a few days in Tabuk, whose inhabitants made terms with him agreeing to pay poll-tax.” 
 Reference: Sahih Muslim 1479 e, 1475 b In-book reference : Book 18, Hadith 44. USC-MSA web (English) reference: Book 9, Hadith 3511 http://sunnah.com/muslim/18/44
 Ibn Sa’d’s Kitab al-Tabaqat al-Kabir, Translated by S. Moinul Haq (New Delhi: Kitab Bhavan, 2009) Volume 2, 203-204
 The origins of the Islamic State, being a translation from the Arabic accompanied with annotations Geographic and historic notes of the KITAB FUTUH AL-BULDAN of al-Imam Abu’l Abbas Ahmad Ibn Jabir Al Baladhuri, By Phillip Khurti Hitti, PHD, , volume 1, page 92