Osama Bin Laden thought the attacks on the World Trade Center and the Pentagon were justified because one was a central trading place and one was a military target. He says that since the United States was terrorizing Arab countries for a long time, it served them right if women and children were killed in the attacks. He also believed that the terrorist attacks on America were necessary to show the strength of Islam and Allah, to show that Islam could not be beat if there was war. He thought that the US was weak, and the terrorist attacks would make them weaker.
Bin Laden saw the war with the united states as a war of Islam against crusaders. He thought that in order for the United States not to be a target for terrorism, they needed to stop terrorizing Islam countries. They also needed to stop helping Israel against Palestinians. Also, since he saw the war as a religious one, he said that in order for the jihad to stop, people needed to follow the way of Allah. He used religion to explain the terrorist attacks, and to convince his listeners that his readings and the words of religious leaders made them necessary. The Muslims have to fight back against the crusaders.
Osama Bin Laden saw the actions of the United States as a crusade, and therefore it was the duty of all Muslims to fight against the United States. The result would be a fight of Christian against Muslim, or a war of civilizations. It would not be a good idea for the Western world to accept this because it brings religion to the forefront of the discussion, and ignores the fact that not all Muslims are in favor of the kind of Jihad that Bin Laden was trying to promote. If the war becomes Muslim against Christian, then people are forced to choose sides. When religion is brought into an argument, people become passionate and emotions run high. Many people would feel like they had to choose a side. Many would be involved in the fight who may not have been otherwise.