Dissent Channel

I haven't been able to find any easily available archive of the use of this (I wonder if a FOIA request could get that?), but did find some isolated references to a few examples of dissent memos.

Last year there was one with 51 signers objecting to President Obama's hands off policy in Syria, saying that it was overwhelmed by the violence there, and there was one early in the Clinton years with about a dozen signers asking for stronger action against the Serbs. A bunch more appeared to be from individual officers, often at an embassy, objecting to something about the way we were dealing with the country they were serving in.

One article I read implied that there are maybe half a dozen of these a year. The BBC says that they are "not that unusual", but did say that the one disagreeing with Trump's immigration EO was "unprecedented" because it was signed by hundreds of people.