Step 1: Understand the context of the sentence.
The sentence talks about two groups being involved in something for three decades, which implies a struggle or conflict.
"Intent" is the appropriate word, as it suggests a long-standing purpose or goal in which these two groups are engaged, rather than just an action like "fight."
The other options do not fit the context:
"Fight" suggests a physical conflict, but it doesn't fit the "bitter" tone for something that lasts decades.
"Arrangement" refers to something planned, not a conflict.
"Event" suggests a singular occurrence, but the sentence indicates an ongoing issue.
Step 2: Hence, the correct answer is (2) intent.