Muslims are allowed to lie to unbelievers in order to defeat them. The two forms are:
Taqiyya – Saying something that isn’t true.
Kitman – Lying by omission. An example would be when Muslim apologists quote only a fragment of verse 5:32 (that if anyone kills “it shall be as if he had killed all mankind”) while neglecting to mention that the rest of the verse (and the next) mandate murder in undefined cases of “corruption” and “mischief.”
Though not called Taqiyya by name, Muhammad clearly used deception when he signed a 10-year treaty with the Meccans that allowed him access to their city while he secretly prepared his own forces for a takeover. The unsuspecting residents were conquered in easy fashion after he broke the treaty two years later, and some of the people in the city who had trusted him at his word were executed.
Another example of lying is when Muhammad used deception to trick his personal enemies into letting down their guard and exposing themselves to slaughter by pretending to seek peace. This happened in the case of Ka’b bin al-Ashraf (as previously noted) and again later against Usayr ibn Zarim, a surviving leader of the Banu Nadir tribe, which had been evicted from their home in Medina by the Muslims.