initiating wars is unislamic.. has no basis in the teachings of the Qur'an.. Defensive warfare is encouraged; Qur'an repeatedly tells us not to stay passive in the face of aggresssion AND oppression.
Purely from a historic perspective, isn't it true that the expansion of the muslim lands was primarily a result of waging wars on non-muslims? I mean, how much area you can gain by 'defensive wars'? The muslim lands stretched quite far and wide.
A 'defensive war' means you defend yourself when attacked by someone else. Right?
The details may be debated, but the general way was that the muslim ruler will send a diplomat with a letter to the non-muslim ruler, and ask him that we stand for peace and that he should not unfairly treat his muslim population or to embrace Islam (or something similar). The king will be outraged, kill the diplomat, and will ready his forces to battle the muslims. The muslim army will fight them and take over their lands. And then move on to newer countries. Is it a defensive war?
I listed out the 'obligation' placed by the Qur'an with regards to fighting.
Now building an empire and spreading your religious beliefs at the tip of the sword are different topics. One would need an accurate account of history to understand how 'Islamic' all the wars of conquest really were..
According to my understanding the 'pure' Islam died shortly after the demise of the Prophet and what remained was the same tribal quibblings and grudges which divided Muslims and led to duplicate Khilafa's not always ruled by the most pious and religious of all.
Hence chalking down their actions and adventures as an "Islamic Obligation" would be incorrect.