@Francesco, I was about to write to you that because it is the name of a country, the verb is going to be in singular, but I checked this and turns out that it is not that clear cut. Basically either singular or plural would be ok with these kinds of names but there are some conventions. Here is a link to a page discussing this on the website of the Institute of the Languages of Finland: http://www.kielitoimistonohjepankki.fi/ohje/309

Another good alternative to "irrota" in addition to "jättäytyä" is "irrottautua", which is almost the same as "irrota" (to detach), but also implies a conscious effort to do so.

Paljon kiitoksia! I was misled by the fact that "Yhdysvallat" is plural so I would have expected a plural verb, for example "irtosivatkin".

Hi, The basic form is "irrota" which means to separate or to detach. "vaikka ... irtosikin" here means "even though ... separated" I would translate the whole sentence as "The treaty was signed by many states, although the United States withdrew from it" The usage of the word "irrota" is a little bit irregular here and I personally would opt for another word. "Sopimuksen on allekirjoittanut moni valtio, vaikka yhdysvallat jättäytyikin sopimuksesta." "Jättäytyä" means "to not go along" or "to leave oneself out".