Quran 9:123 – “O you who believe! fight those of the unbelievers who are near to you and let them find in you hardness.”
Commentary on the Quran, Tanwir al-Miqbas min Tafsir Ibn Abbas mentions (below) that the above verse was revealed in order to fight the enemy, Banu Qurayza, a Jewish tribe who lived in Medina. They allied with the pagan Arabs of Makkah in order to exterminate the Muslim community. In short, they were the aggressors.
Tanwir al-Miqbas min Tafsir Ibn ‘Abbas
(O ye who believe!) in Muhammad (pbuh) and in the Qur’an (Fight those of the disbelievers who are near to you) the Banu Qurayzah, Banu’l-Nadir, Fadak and Khaybar, (and let them find harshness in you) toughness from you, (and know) O believers (that Allah is with those who keep their duty (unto Him)) Allah helps the believers: Muhammad (pbuh) and his Companions, by making them victorious over their enemies. 
Mualana Muhammad Ali
Because it was they who persecuted the Muslims. The object was to stop persecutions.
123b. So that you do not yield. 
163 I.e., uncompromising with regard to ethical principles. For the general circumstances in which war is permitted, see 2:190-194, 22:39, 60:8-9, and the corresponding notes, as well as notes 7 and 9 on verse 5 of this surah.
The reference to ‘those deniers of the truth who are near you’ may arise from the fact that only ‘those who are near’ can be dangerous in a physical sense or, alternatively, that – having come from afar – they have already approached the Muslim country with an aggressive intent. 
Yusuf Ali commentary
C1374. When conflict becomes inevitable, the first thing is to clear our surroundings of all evil, for it is only evil that we can rightly fight. To evil we must put up a stout and stiff resisitance. Mealy-mouthed compromises are not right for soldiers of truth and righteousness. They are often a compound of cowardice, weariness, greed, and corruptibility. 
 Tanwir al-Miqbas min Tafsir Ibn ‘Abbas HTTP://WWW.ALTAFSIR.COM/TAFASIR.ASP?TMADHNO=0&TTAFSIRNO=73&TSORANO=9&TAYAHNO=123&TDISPLAY=YES&USERPROFILE=0&LANGUAGEID=2
 The Holy Quran Arabic Text with English Translation, Commentary and comprehensive Introduction [Year 2002 Edition] by Maulana Muhammad Ali page 429
 The Message of The Quran translated and explained by Muhammad Asad page 412
 Yusuf Ali commentary http://www.quran4u.com/Tafsiraya/009%20Tawbah.pdf