There has been considerable debate in the international community as to the legality of the forceful actions in Kosovo in 1999, Afghanistan in 2002 and Iraq in 2003 under the United Nations Charter. There has been consensus, however, that the use of force in all these situations had to be both proportional and necessary. Against the background of these recent armed conflicts, this book offers the first comprehensive assessment of the twin requirements of proportionality and necessity as legal restraints on the forceful actions of States. It also provides a much-needed examination of the relationship between proportionality in the law on the use of force and international humanitarian law.