Rosetta was supposed to carry a US miniprobe Champollion along with Philae. Champollion was cancelled in 1996 for lack of funding, but revived as an independent mission to another comet. It would have brought back to Earth first samples from the surface of a comet. The sample-return part was cancelled first and the entire mission was cancelled in 1999 for budgetary reasons.
CRAF was a cometary mission, a sibling craft to Cassini, which was supposed to fly past an asteroid and orbit a comet and send a penetrator to its surface. CRAF was approved in 1990 but cancelled in 1992 after US Congress cut NASA's budget.
NASA's CONTOUR mission was supposed to fly past five comets. It exploded when it left it Earth-Moon system in 2002, probably as a result of a problem with the main engine as contact was lost immediately after it was used first time. Telescopes were able to detect that the probe had disintegrated into several parts.
So US has tried to reach comets and has also successes: Stardust, which was operational from 1999 to 2011, succeeded in flybys of comets Wild 2 and Tempel 1. returning samples from the coma of the former, as did Deep Impact with its flybys of Tempel 1 and Hartley 2. Deep Impact was launched in 2005 and contact was lost in 2013.
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