Answering Christianity Research Center

Im not sure if i have put this in the right section but seeing as its about an empire and this is politics and religion it seems like the proper place. This question has recently been nagging me for a while. After the Prophet (pbuh) died the muslims started a conquest and quickly defeated the byzantines and persians creating the Rashidun Caliphate. I always thought that we were only given permission to fight in self defense but obviously conquest is an offensive type of war. So id like to know is conquest allowed in Islam if it is solely to spread islam or did they somehow go against the teachings of Islam (which i highly doubt but might aswell not rule it out). Dont get me wrong i also know that alot of merchants and traders spread islam as arabs were natural travellers and traders. Thanks

Salam alaykum everyone.
So yeah ive been having a bit of a problem on a hadith my opponent gave me on youtube and asked me to explain the reasons. The Hadith is Ibn Ishaq/Hisham 819 which is the one about the Prophet(pbuh) ordering the killing of two poets in makkah. I tried looking for this hadith on the answering christianity website and also others but couldnt find the reason i have also looked for the chain of narration and stuff to see if its a strong hadith but unfortunately i havent been able to get anywhere. So yeah i would appreciate someone explaning it to me or giving me evidence that its fabricated,strong etc etc. If it IS on the answering christianity website would someone please send the link so i could see it. Thanks guys