Answer by taking a quote from the following article:

Bernard Lewis, FBA (born 31 May 1916) is a British American historian specializing in oriental studies. He is also known as a public intellectual and political commentator. Lewis is the Cleveland E. Dodge Professor Emeritus of Near Eastern Studies at Princeton University. Lewis' expertise is in the history of Islam and the interaction between Islam and the West.

Lewis' influence extends beyond academia to the general public. He is a pioneer of the social and economic history of the Middle East and is famous for his extensive research of the Ottoman archives. He began his research career with the study of medieval Arab, especially Syrian, history. His first article, dedicated to professional guilds of medieval Islam, had been widely regarded as the most authoritative work on the subject for about thirty years. However, after the establishment of the state of Israel in 1948, scholars of Jewish origin found it more and more difficult to conduct archival and field research in the Arab countries, where they were suspected of espionage. Therefore, Lewis switched to the study of the Ottoman Empire, while continuing to research Arab history through the Ottoman archives which had only recently been opened to Western researchers. A series of articles that Lewis published over the next several years revolutionized the history of the Middle East by giving a broad picture of Islamic society, including its government, economy, and demographics.  Lewis argues that the Middle East is currently backward and its decline was a largely self-inflicted condition resulting from both culture and religion, as opposed to the post-colonialist view which posits the problems of the region as economic and political maldevelopment mainly due to the 19th-century European colonization. In his 1982 work Muslim Discovery of Europe, Lewis argues that Muslim societies could not keep pace with the West and that "Crusader successes were due in no small part to Muslim weakness." Further, he suggested that as early as the 11th century Islamic societies were decaying, primarily the byproduct of internal problems like "cultural arrogance," which was a barrier to creative borrowing, rather than external pressures like the Crusades.  In the wake of Soviet and Arab attempts to delegitimize Israel as a racist country, Lewis wrote a study of anti-Semitism, Semites and Anti-Semites (1986). In other works he argued Arab rage against Israel was disproportionate to other tragedies or injustices in the Muslim world, such as the Soviet invasion of Afghanistan and control of Muslim-majority land in Central Asia, the bloody and destructive fighting during the Hama uprising in Syria (1982), the Algerian civil war (1992-98), and the Iran-Iraq War (1980-88).  In addition to his scholarly works, Lewis wrote several influential books accessible to the general public: The Arabs in History (1950), The Middle East and the West (1964), and The Middle East (1995). In the wake of the September 11, 2001 attacks, the interest in Lewis's work surged, especially his 1990 essay The Roots of Muslim Rage. Three of his books were published after 9/11: What Went Wrong? (written before the attacks), which explored the reasons of the Muslim world's apprehension of (and sometimes outright hostility to) modernization; The Crisis of Islam; and Islam: The Religion and the People.

what was his biggest accomplishment?
Lewis wrote several influential books