I want to know what portion of total United States petroleum
consumption is consumed by the military (all branches of the armed
forces) and I'd like to know where to find out more details about
that, such as: how much of that (military consumption)  is for
transportation as opposed to other uses; and what are the sources of
that petroleum (what countries, companies).

Dear blotto, 

According to the US Energy Information Administration (EIA), the US
military consumed 346,060,000 gallons of petroleum in 2001 (the latest
year published). You could find more information about it, plus a
division between different sorts of petroleum in the EIA site (EIA,
"Military, Off-Highway, and All Other Sales by PADD and State"
http://www.eia.doe.gov/pub/oil_gas/petroleum/data_publications/fuel_oil_and_kerosene_sales/current/pdf/table24.pdf
- this document is a PDF document and requests an Adobe Acrobat
Reader, which could be downloaded for free).

In 2001, the US consumed 20.4 million barrels (about 856,800,000
gallons) per day, which makes it 312,732,000,000 gallons per year.
A general division of the consumption could be fund here
http://www.ott.doe.gov/facts/archives/fotw160.shtml

The Annual Enegry Review of the EIA could answer other related
questions.

See also EIA, Annual Energy Outlook 2002 with Projections to 2020
http://www.eia.doe.gov/oiaf/archive/aeo02/index.html

More information on US military petroleum consumption: 
"iraq, cost of war, military buildup, kuwait - FCNL" -
http://www.fcnl.org/issues/int/sup/iraq_accounting101-02.htm
Oil Security
http://www.wri.org/cpi/pubs/oil-cni.html 

I hope that answers your question. My search strategy was to search
for military and the term "oil consumption" on Google. Please contact
me if you have any clarification reqeusts on this question. I'd be
pleased to clarify the answer before you rate it.