It's hardly a precedent, probably half of the countries worldwide have been formed by seceding from some other country against its will. U.S. would be in this half.
It's the first country to do so under foreign military presence since UN inception.
The only precedents of unilateral secession were Slovenia and Croatia from Yugoslavia and Bangladesh from Pakistan but none did so under foreign military presence.
All of the Arab countries have basically seceded from Ottoman empire under an occupation of this or that European country.
"Unilaterally" is not easy to define. Sometimes there is a long and violent struggle for independence and the metropole eventually gives in and signs some paper, sometimes it is stubborn and doesn't sign anything - the difference is not that important in my opinion.
The only precedents of unilateral secession were Slovenia and Croatia from Yugoslavia and Bangladesh from Pakistan but none did so under foreign military presence.