The presence of a foreign military power in a country may take the form of access to and use of military facilities (usually in the form of a military, paramilitary or clandestine base), or the actual presence of organized units of military personnel in foreign countries, or the deployment and permanent activity of fleets outside their own territorial waters. The military presence of the great powers in foreign territories is part of the mutual confrontation between the two opposing military and political blocs. In some cases they also serve to prevent political changes which are undesirable to the deploying country. The host country may have no jurisdiction over the military base and little control over the military or clandestine personnel in the country.
It is estimated that more than 20 countries maintain military forces and bases abroad. Among these, the USA, USSR, UK and France hold dominant roles, both in numbers of military forces and naval presence, and impact on the world's military balance and strategic planning. It is recognized that many bases also exist for use by big powers in wartime, under secret agreements, in which no foreign troops are deployed in peacetime. Covert (legal) and clandestine (illegal) presences of fighters, trainers, technical advisers, equipment maintainers, or military and civilian intelligence agents, evade the communication networks of the host country, whether pro- or anti-government, in order to pursue their own purposes. For example, the USA interventions in Latin America are frequently covert, and its base in Panama for many years served as the springboard for interventions to the south.