That’s a very good question, actually. Congress put the final nail in Vietnam when it cut off funding. Congress voted on and approved a cutoff of funds for Somalia, but Clinton was already on the way out. Congress ended funding for the Contras, but then Oliver North went and ignored the law. I’m not 100% certain, but I believe there was debate about a funding cutoff in relation to the 1982 Lebanon deployment…
Give me a little more time and I can dig up some facts, but to my recollection, cutting off funds is something only attempted when we’re pretty much on the way out anyways.