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A Saudi airstrike on what it said was a weapons shipment in Yemen on Tuesday marked the most significant escalation between Riyadh and Abu Dhabi to date. Once the twin pillars of regional security, the two Gulf heavyweights have seen their interests diverge on everything from oil quotas to geopolitical influence.
Here is a timeline of how the relationship has evolved:
As the Arab Spring spreads, the two form a unified front against extremist movements, deploying joint forces to Bahrain to quell an uprising and coordinating support for Egypts 2013 military overthrow of the Muslim Brotherhood government.
March, 2015:
