Subtract rows in indexed dataframe

Question

I am currently working with this data frame. It is indexed by year and Country. What I would like to do is to subtract the values for "military_exp" for year 2011 and the values for "military_exp" for year 2010. Is there a way of doing this?

                       gdp_share    military_exp
year    Country                     
2010    USA            5.0  768465792.0
        China          2.0  138028416.0 
        Korea          3.0  31117330.0  
        Russia         4.0  43120560.0  
2011    USA            5.0  758988352.0
        China          2.0  149022400.0
        Korea          3.0  31543720.0
        Russia         3.0  46022120.0

Answer 1

IIUC

df.groupby(level=1)['military_exp'].diff()
Out[195]: 
year  Country
2010  USA               NaN
      China             NaN
      Korea             NaN
      Russia            NaN
2011  USA        -9477440.0
      China      10993984.0
      Korea        426390.0
      Russia      2901560.0
Name: military_exp, dtype: float64

Update

df.loc[2011,'military_exp']-df.loc[2010,'military_exp']
Out[197]: 
Country
USA       -9477440.0
China     10993984.0
Korea       426390.0
Russia     2901560.0
Name: military_exp, dtype: float64