Hey Mexica, Capitals increase their Military Power every week based on the amount of Resources or WWO-Influence spent in Military Units. You can find all information about Capital Military Power in the Leaderboard Guide ( http://www.worldwaronline.com/guide/leaderboard ) and the Capital Military Power section ( http://www.worldwaronline.com/country/military_power ).
You can check the amount of Resources Spent by hovering over the Resource Bar of your own country in the Capital Military Power section. For example, Mexico has, at the time of this comment, spent over 182 Million Resources in Military Units.
Additionally, given that 4 Capitals will get promoted per week until the Season ends, Mexico's Capital is currently expected to become Silver Military Power on the 4th of January :)