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"Mexicali Blues" is a song from Bob Weir's 1972 Ace solo album that, like the rest of the material on that record, was de facto by the Grateful Dead. Indeed, it appears on the 1974 Skeletons from the Closet: The Best of Grateful Dead compilation.
"Mexicali Blues" was written by Bob Weir and lyricist John Perry Barlow. This was the first songwriting collaboration for Weir and Barlow. Barlow has noted that Weir had an idea for a "cowboy song" and asked Barlow to write the lyrics after Robert Hunter declined. Weir would soon switch to using Barlow rather than Hunter for the bulk of his songwriting.
The song concerns a man who had recently ridden to Mexicali, Mexico from Bakersfield, California. There over a bottle of booze, he thinks back upon his meeting a girl named "Billie Jean" and falling under her spell; she later appeals to the narrator to shoot a stranger when she tells him that unless he uses his gun to prevent it, the stranger will take her away. He does shoot and kill the stranger (who never even drew his gun), and then flees to Mexico rather than face hanging for his crime.
The song echos "El Paso" by Marty Robbins, in which a cowboy shoots a man in a jealous rage over a Mexican girl and then flees to avoid hanging.