Reuters
US mechanized infantry troops at an official opening ceremony for joint US-Georgian exercise Noble Partner 2015 at the Vaziani training area outside Tbilisi, Georgia, May 11, 2015.
Head-to-head comparisons of militaries are hard to make. Each force brings different capabilities to the table and excels in different environments.
Global Firepower's 2017 Military Strength Ranking tries to compensate for those differences by drawing on more than 50 factors - among them diversity of weapons, available manpower, logistical capacity, and industrial base - to assign a Power Index score to 133 countries.
The number of troops and available personnel a country can draw on are central components of its military power. The list below is composed of the 15 militaries with the most active-duty personnel - the "ready to fight" units that would be the first to see combat: