I still don't see any specific correlations between the Tau Empire and Modern United States that couldn't be drawn with any 'x' and 'y' because of how broad the comparisons are.
More food for thought: Japan entered WWI in order to seize the opportunity of Germany's distraction/war in Europe and expand it's sphere of influence in China and the Pacific (by conquering German territory in said areas). The Tau Empire (more precisely, Aun'Va) executed the latter half of the Damocles Crusade (where Farsight was made an 'O and gained notoriety) when the Imperium of Man redeployed many of it's forces to more central worlds to battle Hive Fleet Leviathon (or Behemoth, I forget which) and expanded it's sphere of influence. Art mirroring history, here.
“Though he has his suspicions that it is the Dawn Blade that has prolonged his lifespan to such a degree, if Farsight ever found out the horrible truth, he would likely end his own life in ritual suicide then and there.”
Excerpt From: Games Workshop Ltd. “FARSIGHT ENCLAVES.” iBooks.
Ritual Suicide = Seppuku.