The previous post is really strong. I would only add that industrialization had reached a dominant level in Europe and the desire to add to this base of wealth helped to expand European borders into conquering and overtaking nations. The idea that wealth can be enhanced through taking weaker nations was something that was highly seductive to the European leaders who saw imperialism as a way to consolidate wealth and greater power. At the same time, the rise of the Social Darwinist gospel helped to provide the intellectual justification behind imperialism. The misreading of the scientist that suggested that nature is a "winner take all" setting was one that justified European conduct with enslavement, the taking of indigenous people and natural resources, and outward occupation of countries. The idea of being able to expand because one had the power to do so helped to make imperialism something easier to swallow. Finally, given the fact that many of the nations taken over by the Europeans were nations of people with darker skin colors, one can see racism as a reason for imperialism. The inherent belief that White Europeans were superior on a racial level to the people being conquered helped to justify the need to control and subdue "the darker peoples," in explaining away imperialism.
Europeans had a desire to expand their empires and they wanted to control areas where natural resources were readily available. The existance of European technology such as the telegraph, machine guns, railroads, and steam engines allowed the Europeans to invade and control Africa while still being connected to their homeland. The discovery of quanine, used to cure malaria, gold, and diamonds propelled the desire to imperialize Africa.
The weakened Ottoman Empire, based in Turkey, made the imperialization of the Muslim area possible. Europeans, propelled by greed, were eager to acquire more land and raw materials for refining and production. The Ottoman Empire eventually fell and the land was divided between surrounding nations.
British imperialism was also possible in India due to the weakened Mughal Empire. The British East India Company owned large quantities of land in India and through default and debt, the British were able to take over India and use the land and farmers for cash crops. The British East India Company improved India by adding a railway system, telephone and telegraph lines, built hospitals and schools, and canals, dams, and bridges. While enjoying the benefits of British rule, laws were passed thereby forcing the Indian people into slave labor.