Some duties in Islam are special, e.g. Jihad is one of the major duties of Muslims'.
The "lesser jihad" is the physical struggle against the enemies of Islam. This physical struggle can take a violent form or a non-violent form.
In the Qur'an, God (Allah) encouraged MulimsMuslims to kill Islam's enemies in numerous Ayahs. This is the main reason for the Muslim'sMuslim conquers in the first 200 years of Islam.
Why it is unique to Islam?
In the comments, there is a claim that many other followers of other religions killed the others. This is of course true but irrelevant here. Why such comparisons are wrong? What we see in Islam, very explicitly is written in Quran, it is not a word of a random cleric. This is one of the most fundamental duties in Muslim's holy book. And in one way or another, it has been encouraged.