Disclaimer: I really like the "bone of contention" option. However, these are alternatives.

1. Oxford Spanish Dictionary. Caballo de batalla: Controversial issue: (en discusión).


2. New Oxford American Dictionary. Tug of war: figurative; a situation in which two evenly matched people or factions are striving to keep or obtain the same thing: a tug of war between builders and environmentalists.

another possibility is "inflammatory issue" but reading about what has actually happened (see below), what comes across is that Morocco and the POLISARIO feel extremely sore with one another, and the POLISARIO, being the underdog, has especial reason to feel betrayed by Morocco.

Morocco and POLISARIO formally agreed to a United Nations - sponsored cease-fire in 1991, which was to be followed by a UN-supervised referendum in 1992, giving Sahrawis the option of independence or incorporation into Morocco. The plan was POLISARIO's best hope, but it was also risky. Its winner-take-all formula stipulated that defeat would necessitate POLISARIO's disbanding. As it happened, the vote never took place, owing to the two parties' persistent failure to reach agreement on the list of eligible voters. In 2001, Morocco's unhappiness with the revised voter rolls, and the UN Security Council's unwillingness to impose the referendum by force, led UN mediator James A. Baker to propose delaying the referendum in favor of a five-year period of autonomy for the region, to be followed by a referendum with an expanded voter list. POLISARIO initially resisted the plan. However, in 2003, under the prodding of Algeria, it assented, and the UN Security Council endorsed the plan. Fearing that its claim to sovereignty over the territory, Morocco rejected the plan, and the matter remained unresolved.
http://www.answers.com/topic/polisario-mideast-in-encycloped...

sounds better than bone of.