Fight those of the disbelievers who are close to you The Order for Jihad against the Disbelievers, the Closest, then the Farthest Areas
(Surah At-Taubah 9: 123)
Allah commands the believers to fight the disbelievers, the closest in area to the Islamic state, then the farthest. This is why the Messenger of Allah (Sallallahu Alayhi wa Sallam) started fighting the idolators in the Arabian Peninsula. When he finished with them and Allah gave him control over Makkah, Al-Madinah, At-Ta'if, Yemen, Yamamah, Hajr, Khaybar, Hadramawt and other Arab provinces, and the various Arab tribes entered Islam in large crowds, he then started fighting the People of the Scriptures.
He (Sallallahu Alayhi wa Sallam) began preparations to fight the Romans who were the closest in area to the Arabian Peninsula, and as such, had the most right to be called to Islam, especially since they were from the People of the Scriptures. The Prophet (Sallallahu Alayhi wa Sallam) marched until he reached Tabuk and went back because of the extreme hardship, little rain and little supplies. This battle occurred on the ninth year after his Hijrah. In the tenth year, the Messenger of Allah was busy with the Farewell Hajj. The Messenger (Sallallahu Alayhi wa Sallam) died eighty-one days after he returned from that Hajj, Allah chose him for what He had prepared for him [in Paradise].
After his death, his executor, friend, and Khalifah, Abu Bakr As-Siddiq, (Radhi Allahu Anhu), became the leader. At that time, the religion came under attack and would have been defeated, if it had not been for the fact that Allah gave the religion firmness through Abu Bakr, who established its basis and made its foundations firm. He brought those who strayed from the religion back to it, and made those who reverted from Islam return. He took the Zakah from the evil people who did not want to pay it, and explained the truth to those who were unaware of it. On behalf of the Prophet (Sallallahu Alayhi wa Sallam), Abu Bakr (Radhi Allahu Anhu) delivered what he was entrusted with. Then, he started preparing the Islamic armies to fight the Roman cross worshippers, and the Persian fire worshippers. By the blessing of his mission, Allah opened the lands for him and brought down Caesar and Kisra and those who obeyed them among the servants. Abu Bakr (Radhi Allahu Anhu) spent their treasures in the cause of Allah, just as the Messenger of Allah (Sallallahu Alayhi wa Sallam) had foretold would happen.
This mission continued after Abu Bakr (Radhi Allahu Anhu) at the hands of he whom Abu Bakr chose to be his successor, Al-Faruq, the Martyr of the Mihrab, Abu Hafs, `Umar bin Al-Khattab, (Radhi Allahu Anhu). With `Umar, (Radhi Allahu Anhu) Allah humiliated the disbelievers, suppressed the tyrants and hypocrites, and opened the eastern and western parts of the world. The treasures of various countries were brought to `Umar (Radhi Allahu Anhu) from near and far provinces, and he divided them according to the legitimate and accepted method. `Umar (Radhi Allahu Anhu) then died as a martyr after he lived a praise worthy life.
Then, the Companions among the Muhajirin and Ansar (Radhi Allahu Anhum) agreed to chose after `Umar, `Uthman bin `Affan, (Radhi Allahu Anhu) Leader of the faithful and Martyr of the House. During `Uthman's reign, Islam wore its widest garment and Allah's unequivocal proof was established in various parts of the world over the necks of the servants. Islam appeared in the eastern and western parts of the world and Allah's Word was elevated and His religion apparent. The pure religion reached its deepest aims against Allah's enemies, and whenever Muslims overcame an Ummah, they moved to the next one, and then the next one, crushing the tyranical evil doers. They did this in reverence to Allah's statement, O you who believe! Fight those of the disbelievers who are close to you,
Allah said next, and let them find harshness in you,
meaning, let the disbelievers find harshness in you against them in battle. The complete believer is he who is kind to his believing brother, and harsh with his disbelieving enemy. Allah said in other Ayah, Allah will bring a people whom He will love and they will love Him; humble towards the believers, stern towards the disbelievers...[Surah Al-Ma’idah 5: 54], Muhammad is the Messenger of Allah. And those who are with him are severe against the disbelievers, and merciful among themselves.[Surah Al-Fath 48: 29],
and, O Prophet! Strive hard against the disbelievers and the hypocrites, and be harsh against them.[Surah At-Taubah 9: 73]
Allah said, And know that Allah is with those who have Taqwa,
meaning, fight the disbelievers and trust in Allah knowing that Allah is with you if you fear and obey Him. This was the case in the first three blessed generations of Islam, the best members of this Ummah. Since they were firm on the religion and reached an unsurpassed level of obedience to Allah, they consistently prevailed over their enemies. During that era, victories were abundant, and enemies were ever more in a state of utter loss and degradation.
However, after the turmoil began, desires and divisions became prevalent between various Muslim kings, the enemies were eager to attack the outposts of Islam and marched into its territory without much opposition. Then, the Muslim kings were too busy with their enmity for each other. The disbelievers then marched to the capital cities of the Islamic states, after gaining control over many of its areas, in addition to entire Islamic lands. Verily, ownership of all affairs is with Allah in the beginning and in the end.
Whenever a just Muslim king stood up and obeyed Allah's orders, all the while trusting in Allah, Allah helped him regain control over some Muslim lands and took back from the enemy what was compatible to his obedience and support to Allah. We ask Allah to help the Muslims gain control over the forelocks of His disbeliever enemies and to raise high the word of Muslims over all lands. Verily, Allah is Most Generous, Most Giving.
Ameen thumma AmeenSource: Tafsir ibn Kathir (Abridged)