His work at Princeton was delayed when he was drafted into the Army during the Vietnam War, where he came to admire the drill sergeant to whom he was assigned for basic training.

[2][4] With his thesis advisor Curtis Callan, Caswell embarked on an ambitious program for the summation of Feynman loops in order to calculate elementary particle properties.

[5] His graduate career coincided with the synthesis of gauge symmetry and renormalization group ideas, in which he himself made several pioneering contributions, the highpoint of which was his 1972 calculation of the beta function to two-loop accuracy.

According to Physics Today, this effort "required unusual courage and determination, since the calculation simultaneously features all the notorious subtleties of gauge invariance, overlapping divergences, and renormalization.

"[4] In the age of punchcards, FORTRAN and paper output, Caswell felt that pure hand calculation was excruciating to perform and impractical to check.

In 1985, he joined a major classified defense technology project and rose to a position of technical responsibility, directing a team of more than 100 scientists.