On the warpath
Intent on a confrontation or fight.
'War path' was originally written as two words and, following the usual hyphenation phase, that is, as 'war-path', it is now more commonly seen as the single word 'warpath'.
The 'war path' was the literal 'path to war' taken by native Americans (who were variously referred to in early citations of this phrase as Indians, red-skins or savages) when travelling to an enemy's territory to engage in battle. In America in So Many Words, 1997, Metcalf and Barnhart state that the name was used in 1755 on 'a map' - "Canoes may come up to the Crossing of the War Path". Unfortunately, they don't cite their source, although the book is well-researched and I've no reason to doubt their assertion. The phrase was unquestionably in use in the USA twenty years later, when James Adair included it in The history of the American Indians, 1775 and qualifies as one of the very earliest American phrases:
"I often have rode that war path alone."
The 'war path' was also sometimes called the 'warrior's path', as in A Sketch of the History of Wyoming, by Isaac Chapman, 1830:
"He commenced his march by way of Fort Allen on the Lehigh near the Water-gap, and thence by the warrior's path to Wyoming."
It took a little while for the phrase to take on its present-day figurative meaning. The process of change from the literal began with this simile in the Cyclopaedia of Wit and Humor, by William Burton, 1859:
"Tell not such a tale to the seaman,... nor to the author,... publishing at his own cost, the critics, like savages, come out on the war path, track him by the print he makes, and then scalp him."
The process was complete by 1880, when Mark Twain (S. L. Clemens) used the phrase with no Indians in sight, in the travelogue A tramp abroad:
"She was on the war path all the evening."
The phrase is still used when tribes go to war, as in the recent (September, 2007) 'Microsoft on the warpath' headlines that were used to report the so-called 'Portal war' between Microsoft and Google.
See other phrases that were coined in the USA.