Ruling on defending the honour of the kuffaar
Question

What is the ruling on defending the honour of the kuffaar? One day I slandered the honour of a kaafir and one of the Muslims responded and defended the honour of this kaafir. What is the ruling on defending their honour?.
Praise be to Allah.Firstly: 
It is not the attitude of a Muslim to insult others, slander them and impugn their honour. The Prophet (peace and blessings of Allaah be upon him) said: “The believer is not given to cursing, slandering or obscene and foul speech.” Narrated by Ahmad, 3948; al-Tirmidhi, 1977; classed as saheeh by al-Albaani in Saheeh al-Tirmidhi. 
The Prophet (peace and blessings of Allaah be upon him) told people not to insult a Jew who deserved to be insulted, and he said that the reason for that is that Allaah does not like obscene talk. 
Al-Bukhaari (6401) narrated from ‘Aa’ishah that the Jews came to the Prophet (peace and blessings of Allaah be upon him) and said: “Al-saam ‘alayk (death be upon you). He said: “Wa ‘alaykum (and also upon you).” ‘Aa’ishah said: “Death be upon you, may Allaah send His curse and wrath upon you.” The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Take it easy, O ‘Aa’ishah! You should be kind and gentle, and beware of harshness and foul speech.” She said: “Did you not hear what they said?” He said: “Did you not hear what I said? I responded to them, and my du’aa’ concerning them was answered, but their du’aa’ concerning me was not answered.” 
According to a report narrated by Muslim (2165), ‘Aa’ishah noticed what they said and insulted them, and the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Enough, O ‘Aa’ishah, for Allaah does not like foul speech and foul language.” 
Al-Haafiz ibn Hajar (may Allaah have mercy on him) said: 
It seems that the Prophet (peace and blessings of Allaah be upon him) did not want her tongue to become accustomed to foul speech so he denounced her for going to extremes in insulting them. 
End quote from Fath al-Baari, 11/43. 
Secondly: 
One of the following two scenarios most apply in the case of a kaafir: 
Either he is in a state of war against the Muslims, in which case he has no sanctity (i.e., he is not protected); 
Or he has a peace treaty with the Muslims or is living under Muslim rule, in which case his life, honour and wealth are protected, and it is not permissible to transgress against him. The Prophet (peace and blessings of Allaah be upon him) warned against transgressing against or wronging a kaafir whose rights are protected, as he said: “Whoever does wrong to a mu’aahad (a kaafir who has a peace treaty with the Muslims), or tries to put him down, or burdens him with more than he can bear, or takes something from him without his consent, I will be his opponent on the Day of Resurrection.” Narrated by Abu Dawood, 3052; classed as saheeh by al-Albaani in Saheeh Abi Dawood. 
End quote from ‘Awn al-Ma’bood. 
Al-San’aani said in Subul al-Salaam (2/663), concerning the words of the Prophet (peace and blessings of Allaah be upon him), “Trading insults with a Muslim is evildoing”: 
It may be understood from the word “Muslim” here that it is permissible to insult a kaafir, but if he is a mu’aahad then it is an offence to him, and it is forbidden to offend him, so we should not follow what may be understood from this hadeeth in such a case. But if the kaafir is one who is at war with the Muslims, then it is permissible to insult him because he has no protection. End quote. 
It says in al-Mawsoo’ah al-Fiqhiyyah (24/142): 
For a Muslim to slander or insult a dhimmi (a non-Muslim living under Muslim rule) is a sin, and a Muslim is to be punished if he insults a kaafir. Al-Shaafa’i said: It makes no difference if he is alive or he is dead and is known to have died in kufr. Al-Buhooti, one of the Hanbalis, said: The punishment is because transgressing the limits set by Allaah. End quote. 
It also says in al-Mawsoo’ah al-Fiqhiyyah (30/139): 
Those who are living under a peace treaty or covenant with the Muslims are entitled to be safe with regard to their lives, their wealth and their honour. The ruler should protect them from everyone among the Muslims and others who wishes to harm them, and they should not be wronged so long as the covenant between them and the Muslims is in effect. End quote. 
If the slander was an accusation of zina (adultery, fornication), then that makes matters even worse. The scholars (may Allaah have mercy on them) have stated that the one who does that is to be punished. 
See al-Mughni. 9/48; al-Furoo’, 6/108; al-Insaaf, 10/203; Nasab al-Raayah, 4/174. 
The point is that whoever defends the honour of a kaafir, by refuting what is said about him wrongfully, is doing no wrong, in fact he is doing the right thing. It is not the way of a Muslim to slander people’s honour. If he also tells him to stop slandering and insulting others, he is doing good and will be rewarded in sha Allaah. 
And Allaah knows best.