A learner's guide
Parsing a sentence according to the question:
/Several previous deals //to end the fighting in Idlib// have collapsed.
the infinitive may function as a subject, direct object, subject
complement, adjective, or adverb in a sentence. Although an infinitive
is easy to locate because of the to + verb form, deciding what
function it has in a sentence can sometimes be confusing.
Purdue Writing Lab
to end the fighting in Idlib=can be viewed as a subject complement, used in an infinitive phrase. A subject complement says something about the subject of the sentence.
several previous deals=subject of the sentence
have collapsed=verb
If you remove the infinitive phrase, you still get a clause with meaning and this makes the verb stand out:
/Several previous deals have collapsed/.
The main verb is have collapsed.