In February 1998, the 40-year-old Saudi exile Usama  Bin Laden and a fugitive Egyptian physician, Ayman al Zawahiri, arranged from their Afghan headquarters for an Arabic newspaper in London to publish what they termed a fatwa issued in the name of a "World Islamic Front." A fatwa is normally an interpretation of Islamic law by a respected Islamic authority, but neither  Bin Laden, Zawahiri, nor the three others who signed this statement were scholars of Islamic law. Claiming that America had declared war against God and his messenger, they called for the murder of any American, anywhere on earth, as the "individual duty for every Muslim who can do it in any country in which it is possible to do it." Three months later, when interviewed in Afghanistan by ABC-TV,  Bin Laden enlarged on these themes. He claimed it was more important for Muslims to kill Americans than to kill other infidels." It is far better for anyone to kill a single American soldier than to squander his efforts on other activities," he said. Asked whether he approved of terrorism and of attacks on civilians, he replied:"We believe that the worst thieves in the world today and the worst terrorists are the Americans. Nothing could stop you except perhaps retaliation in kind. We do not have to differentiate between military or civilian. As far as we are concerned, they are all targets." Note: Islamic names often do not follow the Western practice of the consistent use of surnames. Given the variety of names we mention, we chose to refer to individuals by the last word in the names by which they are known: Nawaf al Hazmi as Hazmi, for instance, omitting the article "al" that would be part of their name in their own societies. We generally make an exception for the more familiar English usage of "Bin" as part of a last name, as in  Bin Laden. Further, there is no universally accepted way to transliterate Arabic words and names into English. We have relied on a mix of common sense, the sound of the name in Arabic, and common usage in source materials, the press, or government documents. When we quote from a source document, we use its transliteration, e.g.,"al Qida" instead of al Qaeda. Though novel for its open endorsement of indiscriminate killing,  Bin Laden's 1998 declaration was only the latest in the long series of his public and private calls since 1992 that singled out the United States for attack. In August 1996,  Bin Laden had issued his own self-styled fatwa calling on Muslims to drive American soldiers out of Saudi Arabia. 
What reasons does  Bin Laden give for issuing a fatwa on all Americans?
He claimed that American had declared war against God [Allah] and his messenger [Mohammed] and that Americans are the worst thieves and terrorists in the world