The main aim of this study is to shed light on the major factors affecting the performance of the Muslim military in five important battles fought during the first two decades of Islam (2/624-16/638). The study comprises an introduction, seven central chapters, and a conclusion (an Arabic-English glossary is placed at the end of the study). The first chapter includes a general introduction which incorporates an explanation of the method to be followed in the study. It provides a review of the study's primary and secondary sources and of the Arabic literature on the subject of warfare and military activities. It also explains the nature and scope of the study and closes with an outline of the organisation of the chapters. The second chapter provides a brief background account of the region prior to Islam and considers in particular the region of al-Ilijäz. In addition it illustrates some of the main features of the neighbouring powers in the north and south of Arabia, and of the two great empires that still dominated the region. The third chapter narrates the significant events of the battles under discussion, i. e. Badr, Uhud, al-Khandaq, al-Yarmiik, and al-Qädisiyyah, thus providing a reference for the subsequent chapters. The fourth chapter investigates the factor of Islamic military doctrine, its role in the performance of the Muslim armies, and its importance for the conduct of the five battles. The fifth chapter examines the factor of Islamic military leadership, its impact upon the performance of the Muslim armies, and how it was exercised during the battles under study. The sixth chapter discusses the factor of Islamic military intelligence, its task in relation to the performance of the Muslim armies, and how it was conducted in the battles in question. The seventh chapter considers the factor of the adversaries' strengths and weaknesses using both narrative and analysis. This chapter provides a necessary examination of the nature of the forces confronting the Muslim armies. The eighth chapter focuses on the factor of Islamic military strategy, its importance for the performance of the Muslim armies, and its implementation in the five battles. The ninth chapter summarises and discusses the study's most important findings and draws conclusions from them, indicates to what extent the study's principal aims have been achieved, and makes suggestions for future research.