Which invasion do you mean? Afghanistan has been invaded many times, and in fact its boundaries and legitimate government have almost always been in dispute. Between the Mughal rulers of South Asia, the Russian Tsars and later Communist government, British Empire and now America and the Western Coalition, everyone seems interested in this territory.
The first generally recognized invasion of Afghanistan was made by Alexander the Great in 330 BC as part of his string of conquests. Among the cities conquered was Herat.
Afghanistan was invaded from the west by the Arabs under the Abbasid Dynasty, causing the conversion of most of its inhabitants to Islam. Later, Afghanistan was invaded twice from the north and east by the Mongols (once by Genghis Khan, once by Timur) in a drive to conquer both India and the heartlands of Dar al-Islam.
The Soviet Invasion of Afghanistan happened in 1979, prompting a Western boycott of the Olympics, and kickstarting US funding for radical, armed islamic resistance groups. Mujahadeen eventually succeeded in forcing the Soviets out, in the USSR's most humiliating military defeat, which some say was the beginning of the end for Russian communism.
Most recently, the U.S. invasion of Afghanistan was a stated attempt to capture Osama Bin Laden, the terrorist they claim masterminded the bombing of the World Trade Center on Sept. 11, 2001. Although they did not capture him, they succeeded in toppling the intensely radical Islamic government of the Taliban, whose cruelty had won few friends. The Taliban leadership also survived in hiding and continues, along with many other factions, to keep Afghanistan unstable.
