Al-Baqarah: 194
"The sacred month for the sacred month..."
The polytheists fought them in the year of al-Hudaybiyyah during the sacred month, which was Dhu al-Qa'dah. So, it was said to them when they went out for the 'Umrat al-Qada' (the compensatory pilgrimage) and were averse to fighting—which was also in Dhu al-Qa'dah:
"The sacred month for the sacred month"
Meaning: This month is for that month, and the violation of its sanctity is for the violation of its sanctity. It means: violate its sanctity against them just as they violated its sanctity against you.
"And violations are subject to legal retribution (qisas)"
Meaning: Every sanctity in which retribution is applicable—if someone violates any sanctity whatsoever, retaliate against him by violating a sanctity of his. Since they violated the sanctity of your month, do the same to them and do not hesitate. He emphasized this by saying:
"So whoever has assaulted you, then assault him in the same way that he has assaulted you. And fear Allah"
While you are victorious over those who assaulted you, do not transgress into what is not lawful for you.
"And spend in the way of Allah and do not throw [yourselves] with your [own] hands into destruction. And do good; indeed, Allah loves the doers of good."