The headlines announced a truce between the two countries
indentify the prepositional phrase ?

The preposition here is "between" and the phrase is "between the two countries."

if you're trying to find the prepositional phrase in a sentence and you don't have a list on hand, one trick i learned that usually works is saying to yourself "the bunny is [preposition] the log." i know it sounds silly, but it's a way to test which word in the sentence could be a preposition--"the bunny is announced the log" doesn't make sense, "the bunny is two the log" doesn't make sense either. "the bunny is between the log," however, does work (assuming that the log is split in half, i suppose). remember that prepositions show relationships between nouns/pronouns and other objects in the sentence, and you should be able to answer most questions like these without much trouble.