Not taxable by whom?
The way the tax treaties normally go, a government pension is usually taxable by the country of the government in which it is administered. But the US doesn't generally give you any kind of a break on "foreign" pensions. Normally for the US you have to declare the pension (private or public) and then use foreign tax credits against any US tax liability.
Take a look at IRS publications 575 and 939.
Cheers,
Bev