5. United KingdomThe British Army was organized in 1661 and has been involved in various conflicts including the Napoleonic Wars, Revolutionary War and both world wars.
The British Army has had a significant presence in Northern Ireland and the Balkans while standing beside the United States in the Gulf War in the 1990s as well as the Global War on Terrorism after the attacks of September 11, 2001.
The active duty force of the British Army is more than 100,000 strong, making it the second largest army in the European Union behind France.