I want to shorten the following sentence by making an adjective out of the phrase "parties that have been fighting for many years now":
"The initiative aims at building trust among the parties that have been fighting for many years now."
I thought about adding "long-warring" parties, but I'm not sure if it's correct.
"The initiative aims at building trust among the long-warring parties."
I'm translating a text and there's a specific number of words I can't exceed.
Long-warring is excellent!
Contemporary English seldom uses war as a verb, but the present participle warring is conventional. Long-warring is a very clear, concise way to communicate this idea. And you are right to include the hyphen.
Here's an example from a book written in 1994:
In enacting the Omnibus Budget Reconciliation Act of 1990 (OBRA), the long-warring branches [of the U.S. government] looked ahead to a period of budget tranquility and rapidly declining deficits.
Notice that this example, which is fairly typical, uses long-warring metaphorically: not to describe a literal war, but to describe a long-running disagreement between different branches of government. The metaphor extends to using the word tranquility to describe the expected peace now that the "war" is concluded.
For more examples, see Google Books (which is a great source of information for this kind of question).
Feuding is the word. See feud (Oxford dictionary).
