Response: And your own sources tell you that Constantinople was the Capital of the Byzantine empire, the same empire who initiated the very first crusade. Thus it was an act of defense, as it was the Byzantine empire who strucj first and continued to make attacks. Even when they lost, they never wished to enter into any treaty of peace. Further proof of the fact that it was defense is that even in the present day, we can see the feeble muslim countries, as a result of the relentless attacks of the Byzantine empire crushing the Ottoman empire as it adapted to the new western world we know today.
Please bother to do the most basic research. The first crusade started in the year 1096 the first two islamic sieges of Constantinople happened in the years 674-678 and 717-718, hundreds of years prior to the crusades. Also the first crusade by your definition was a defensive campaign since it was lauched by the Roman pope after an appeal for aid from the Byzantine emperor.
http://en.wikipedia.org/wiki/First_Crusade
All this information is easily obtainable and you making such basic mistakes as dates in favor of your ingrained indoctrination is troubling. Please take the time to read some of the links which I have provided in that they will give you background information on the subject at hand if you do not already know anything about it.
Also your characterization of the Ottoman empire is mistaken. Yes by the time WW1 came around they were known as the sick man of Europe and for good reason. You have to realize the Ottoman empire of 1918 was not the Ottoman empire of 1300, at its outset the Ottoman empire was a power to be reckoned with and in fact did conquer Constantinople in 1453, something which the might of the Caliphate could not do after two tries and hundreds of thousands of casualties.
Yet another point the present state of muslim countries is more an outgrowth of the age of empires in Europe than anything Constantinople did in fact many present muslim countries were created by the European powers during the age of empires from the 16th-20th centuries. If anything the state of muslim countries also reinforces my point that an empire must expand or stagnate once the Caliphate could no longer expand it tore itself apart. The Islamic world was once a shining beacon of learning and tolerance but once the expansion stopped so did the learning and tolerance and just as christianity was leaving its own dark age islam was entering one of its own which from what I can see they never exited.