The annual Global Firepower Index ranking of the world's militaries is out, with the US coming in as the strongest. It's not just a matter of counting weapons and soldiers; the index uses more than 50 data points in the final score, including geographical factors, alliances, and logistical flexibility. India, for example, has 4.2 million military personnel, the most of any nation, but it ranks No. 4 in strength. (The US has 2.4 million personnel.) Here are the top 10: