At-Tawba 9:5

Then when the [four] sacred months[6] are over, kill the polytheists wherever you find them, seize them, besiege them and lie in wait for them on every route[7]. But if they repent, establish prayers and give zakah, then set them free, for Allah is All-Forgiving, Most Merciful[8].

[6] i.e., Muharram, Rajab, Dhul-Qa‘dah and Dhul-Hijjah.
[7] Some prominent scholars of Islam are of the opinion that this applies especially to combatant disbelievers who pose a threat to Muslims.
[8] The next few verses reveal who these really are and why they deserve such retaliatory measures.

And when the inviolable months[464] have passed, then kill the polytheists wherever you find them and capture them and besiege them and sit in wait for them at every place of ambush. But if they should repent, establish prayer, and give zakāh, let them [go] on their way. Indeed, Allāh is Forgiving and Merciful.

[464] The four months mentioned in verse 2, in which the disbelievers were allowed free movement within the land. Other scholars have interpreted them as "the sacred months," i.e., Muḥarram, Rajab, Dhul-Qaʿdah and Dhul-Ḥijjah.

5. Then when the Sacred Months (the 1st, 7th, 11th, and 12th months of the Islâmic calendar) have passed, then kill the Mushrikûn (See V.2:105) wherever you find them, and capture them and besiege them, and lie in wait for them in each and every ambush. But if they repent and perform As-Salât (Iqâmat-as-Salât), and give Zakât, then leave their way free. Verily, Allâh is Oft-Forgiving, Most Merciful.[1]

[1] a) (V.9:5) See the footnote of (V.2:193).
b) Narrated Abû Hurairah رضي الله عنه : When the Prophet صلى الله عليه وسلم died and Abû Bakr became his successor and some of the Arabs reverted to disbelief, ‘Umar said, "O Abû Bakr! How can you fight these people although Allâh’s Messenger صلى الله عليه وسلم said, " I have been ordered to fight the people till they say: La ilâha illallâh (none has the right to be worshipped but Allâh), and whoever said La ilâha illallâh will save his property and his life from me, unless (he does something for which he receives legal punishment) justly, and his account will be on Allâh?" Abû Bakr said, "By Allâh! I will fight who ever differentiates between Salât (prayers) and Zakât, as Zakât is the right to be taken from property (according to Allâh’s orders). By Allâh! If they refused to pay me even a kid they used to pay to Allâh’s Messenger صلى الله عليه وسلم, I would fight with them for withholding it," ‘Umar said "By Allâh! It was nothing, but I noticed that Allâh opened Abû Bakr’s chest towards the decision to fight, therefore I realized that his decision was right." (Sahih Al-Bukhâri, Vol.9, Hadîth No 59).

Then, when the sacred months have passed, slay the idolaters wherever ye find them, and take them (captive), and besiege them, and prepare for them each ambush. But if they repent and establish worship and pay the poor-due, then leave their way free. Lo! Allah is Forgiving, Merciful.