I found two indexes which ranked countries by military strength. The Global Firepower index seems to be the one most often used based on the number of times I came across it while searching for articles.
The second index was part of a 2015 report put out by Credit Suisse. It cited as it's sources Global Firepower, SIPRI, and itself.
Countries Ranked by Military Strength: Global Firepower (2014)
|From Business Insider 07/10/14|
The End of Globalization or a more Multipolar World?: Credit Suisse Research Institute, Sept. 2015
Countries Ranked by Military Strength: Global Firepower