Quran 9:73 – “O Prophet! strive hard against the unbelievers and the hypocrites and be unyielding to them; and their abode is hell, and evil is the destination.”
When the above verse says, “strive hard against the unbelievers”, this refers to the expedition of Tabuk, which Prophet Muhammed (p) and his Companions took part in. The Byzantine (Roman) Empire at the time prepared to attack the Muslim community in Arabia. Prophet Muhammed (p) when he heard of this, he got the Muslims ready to engage the enemy. Three early authentic Islamic reports (Hadiths), inform us that the Byzantine started this aggression.
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.” 
Mualana Muhammad Ali
73a. Jahada signifies he strove or exerted himself, and Jihad is the using one’s utmost power in contending with an object of disapprobation (LL). It is in a secondary sense that the word signifies fighting, and it is repeatedly used in the Holy Qur’an in its primary significance. It is a fact that those who professed Islam were never fought against, even though their professions were insincere – as on this occasion and on the occasion of the Battle of Uhud. ‘The correct rendering is that Jihad signifies striving, or exerting oneself, and there is nothing in the word to indicate that this striving is to be effected by the sword or by the tongue or by any other method’ (Rz). The Prophet is commanded here to carry on a Jihad against disbelievers as well as hypocrites. Hence the only significance that can be attached to these words is that he must continue to preach forcibly both to the disbelievers and the hypocrites. 
Islam and Global Dialogue: Religious Pluralism and the Pursuit of Peace
It needs to be stressed that there is nothing inherently violent about Islam as a religion. Indeed quite the reverse. As anyone with any knowledge of Arabic will tell you, the word Islam, which is generally defined as ‘submission to the Will of God’, derives from the same root as salam, ‘peace’. War, in certain circumstances, is a duty, but it can never be ‘holy’ and it is governed by a strict code of conduct that forbids, for example, the killing of women and children. The striving denoted by the verb jahada is primarily spiritual or moral, as in God’s admonition to the Prophet: ‘Strive hard against the deniers of the truth and the hypocrites (9:73). Other passages indicate that the Qur’an itself is the instrument with which believers must strive against those who disbelieve ((25:52; 66:9). As Dr Seyyed Hossen Nasr says, concerning the concept of Jihad, “its translation into ‘holy war’, combined with erroneous notion of Islam prevalent in the West as the ‘religion of the sword’ has helped to eclipse its inner and spiritual significance (1987:28). All external forms of struggle – and these include fighting ignorance and justice. – are worthless if they are not accompanied by the inner struggle, or greater jihad, against evil and the whims of the ego. The Prophet Muhammad is reported to have said: ‘If a man engaged in battle entertains in his heart a desire to obtain out of the war only a rope to tether his camel, his reward shall be fortified.’ A soldier is not a Mujahid if he is motivated by hatred, or revenge, or personal ambition, or the pursuit of worldly gain, and there is a well-kown story that the Prophet Muhammad’s son-in-law Ali Ibn Aby Talib felt obliged to refrain from killing a man who spat at him to avoid the sin of killing his enemy in anger. Only self-defence, in the widest sense, including the defence of religiou freedmom, makes force permissible for Muslims. If people did not have such a right, then ‘monasteries and churches and synogogues and mosques, in which God’s name is much remembered, would surely have been destroyed by now’ (Qur’an, 22:40). However, t is also written: ‘whoever pardons [his foe] and makes peace, his reward rests with God’ (42:40), and ‘if they incline towards peace, inline thou also to it and place thy trust in God’ (8:61). Those who have been unjustly driven from their homelands have a duty to fight (Nasr 1987: 27-33). However, when the opportunity for peace arises, Muslims are encouraged to be forgiven and to seek reconcilation, for mercy and compassion are God’s chief attributes, and it may be that ‘God will brig about affection between you and some of those whom you now face as enemies (60:7).
According to one Prophetic tradition, to settle a dispute between people is more excellent than fasting, charity, and prayer. On the basis of the above definition of a just war, readers can easily figure out which recent and continuing conflicts may legitimately fall into the Jihad category. 
Mohammad Asad Commentary
101 I.e., ‘Do not compromise with them in matters of principle’. Regarding the meaning of the verb Jahada (he strove hard’, i.e., in a righteous cause), see Surah 4, note 122. 
The Holy Qur’an Arabic Text with English Translation & Short Commentary – Malik Ghulam Faird
The imperative Jahid is obviously used here in its spiritual connotation, implying efforts at convincing unbelievers and the waverers, including the various types of hypocrites spoken of in the preceding passages. Although the imperative is addressed in the first instance to the Prophet, it is considered to be morally binding on al believers. 
Sayyid Abul Ala Maududi – Tafhim al-Qur’an – The Meaning of the Qur’an
81 From here begins !he third discourse that was sent down after the expedition to Tabuk.
82 This Command enunciated the change of policy towards the hypocrites. Up to this time, leniency was being shown to them for two reasons. First, the Muslims had not as yet become so powerful as to take the risk of an internal conflict in addition to the one with the external enemies. The other reason was to give trough respite to those people who were involved in doubts and suspicions so that they could get sufficient time for attaining to faith and belief. But now the time had come far a change of policy. The whole of Arabia had been subdued and a bitter conflict with the external enemies was about to start; therefore it was required that these internal enemies should be crushed down so that they should not be able to conspire with the external enemies to stir up any internal danger to the Muslims. And now it had become possible to crush them. As regards !he second reason, these hypocrites had been given respite for a period of nine years to observe, to consider and test the Right Way, and they could have availed of it, if they had any good in them. So there was no reason why any more leniency should be shown to them. Therefore, Allah enjoined the Muslims to treat the hypocrites on one and the same level with the disbelievers and start Jihad against them, and to give up the policy of leniency Grey had adopted towards them and adopt a fine and stern policy instead.
In this connection, it should also be noted that this verse does not enjoin the Muslims to fight with the hypocrites. It merely meant to end the policy of leniency that had hitherto been adopted towards them. This verse enjoined that they were no more to be considered a part and parcel of the Muslim community nor were they to be allowed to take part in the management of its affairs nor consulted about any matter, so that they might not be able to spread the poison of hypocrisy. This changed policy required that the true Believers should expose all those, who adopted a hypocritical attitude and conduct and showed in any way that they were not sincere allies to Allah, His Messenger and the true Muslims. Each and every one of such hypocrites should be openly criticized and reproved so that there should remain for them no more place of honor and trust in the Muslim society: they should be socially boycotted and kept away from the consultations of the Community: their evidence in the courts of law should be regarded as untrustworthy: the doors of offices and positrons of trust should be closed against them and they should be held in contempt in the social meetings. In short; every Muslim should show by his behavior to such a one that there was no place of honor or respect or trust for a hypocrite in the Muslim society. Besides this, if any one of them was found to be guilty of treachery, there should be no connivance at his crime, nor should he be pardoned but openly tried in a court of law and should be duly punished.
This Command was urgently needed at the time it came. It was obvious that in order to save the Muslim Community from fall and degradation, it was essential to purge it of all the internal dangers to its solidarity, because a Community, which nourishes hypocrites and traitors and allows the internal enemies to flourish with honor and security, wall inevitably be doomed to moral degradation and ultimate destruction. Hypocrisy is a plague and a hypocrite is the rat that carries and spreads its germs. Therefore to allow him the freedom of movement in the society is to expose the whole population to the danger of hypocrisy. Likewise, to give a place of honor and prestige to a hypocrite is to encourage many others in hypocrisy and treachery, for this shows that it is not sincerity, true faith and its welfare that count in it. One may flourish and prosper in it even if one verbally professes to be a Muslim and at the same time indulges in dishonesty and treachery. The Holy Prophet has expressed the same thing in a pithy saying. He said, “Whoso honors and respects the inventor of new practices which are un-Islamic, indeed helps to demolish the very structure of Islam.” 
 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
 The Holy Quran Arabic Text with English Translation, Commentary and comprehensive Introduction[Year 2002 Edition] by Maulana Muhammad Ali page 416
 Islam and Global Dialogue: Religious Pluralism and the Pursuit of Peace by Roger Boase Preface xviii
 The Message of The Quran translated and explained by Muhammad Asad page 395
 The Holy Qur’an Arabic Text with English Translation & Short Commentary: By Malik Ghulam Faird page 394
 Sayyid Abul Ala Maududi – Tafhim al-Qur’an – The Meaning of the Qur’an –http://www.englishtafsir.com/Quran/9/index.html#sdfootnote82sym