A military alliance is an alliance that is made with the express purpose of military matters and promises the defense of all of its members.

Historic military alliances 
Medic Wars
Hellenic League - Athens, Lacedemonia, Macedon, others 
Peloponnesian War
League of Delos - Athens, Chios, Samos, Lesbos, others
Peloponnesian League - Lacedemonia, Corinth, Elis, others
Wars of Scottish Independence
Auld Alliance - Scotland, Norway, France
Italian Wars
League of Cambrai - The Papal States, France, Aragon, Holy Roman Empire
Catholic League (Italian) - The Papal States, Venice, England, Aragon, Holy Roman Empire
Schmalkaldic War
Schmalkaldic League - German Protestant States
Anglo-Portuguese Alliance -  England (succeeded by the United Kingdom) and Portugal, since 1373
 Luso-Islamic War
Ethio-Portuguese Treaties - Portugal, Ethiopia
 Thirty Years' War
 Catholic League (German) - Bavaria, other Catholic German States
 Protestant Union - Protestant German States
War of the Grand Alliance (Nine Years War)
League of Augsburg - The Netherlands, Holy Roman Empire, Spain, Austria, other German states
War of the Spanish Succession
The Quadruple Alliance of 1718
Napoleonic Wars
 The Quadruple Alliance of 1814/15 
 First through Seventh Coalitions
The Quadruple Alliance of 1834
Anglo-Japanese Alliance - United Kingdom and Japan
 World War I
the Entente and the Allies - United Kingdom, France, Serbia, New Zealand, Australia, Russia (to 1917), Romania (from 1916), United States (from 1917)
the Central Powers - Germany, Austria-Hungary, Ottoman Empire, Bulgaria
 World War II
the Axis - Germany, Japan, Italy,and many others.
the Allies - United Kingdom and the Commonwealth, France, United States, Soviet Union, Republic of China, many others.
 Cold War
 Western European Union (WEU) - currently in process of merging into CFSP and ESDP (see below)
 North Atlantic Treaty Organisation (NATO) -- United States, Canada, most of Western Europe (still an active alliance)
 Warsaw Pact - Soviet Union,  East Germany, Bulgaria, Hungary, Poland, Czechoslovakia, Romania
 Southeast Asia Treaty Organisation (SEATO) - United States, United Kingdom, France, Australia, Burma, New Zealand, Pakistan, Thailand, Philippines
 Central Treaty Organisation (CENTO) - United States, United Kingdom, Turkey, Iran, Iraq, Pakistan
 ANZUS - United States, Australia, New Zealand (still an active alliance)
 Five Powers Pact - Australia, Malaysia, New Zealand, Singapore, United Kingdom (still an active alliance)

Current military alliances and economic assistance  

 North Atlantic Treaty Organisation (NATO) - United States, United Kingdom, Canada, most of Western Europe and Central Europe, parts of Eastern Europe, and Turkey.
 Collective Security Treaty Organization (CSTO) - Russia, Belarus, Armenia, Kazakhstan, Kyrgyzstan, Tajikistan
 Common Foreign and Security Policy (CFSP) and European Security and Defence Policy (ESDP) of the European Union.
 Five Power Defence Arrangements (FPDA) - United Kingdom, Australia, New Zealand, Malaysia, Singapore.
 ANZUS Treaty (ANZUS) - United States, Australia, New Zealand.
 ECOMOG of Economic Community of West African States Monitoring Group.
 Council for Peace and Security in Central Africa (COPAX) of Economic Community of Central African States.
 Inter-American Treaty of Reciprocal Assistance (Rio Pact) - Most of the countries of the Americas.
 North American Aerospace Defense Command (NORAD) - United States and Canada. NORAD and other defense agreements (such as the Permanent Joint Board on Defense) form the most extensive defense alliance in the world.
 Regional Security System (RSS) - several Eastern Caribbean countries.
 Shanghai Cooperation Organisation (SCO), political, economic and military organisation - China, Russia, India, Pakistan, Kazakhstan, Kyrgyzstan, Tajikistan, Uzbekistan.
Brazil, Russia, India, China, South Africa (BRICS) - economic and military assistance.

References 

Military