Complex sentence: Relations were hurt when American diplomatic cables revealed by the whistle-blowing website Wikileaks in December 2010 suggested that the USA was concerned that Saudi Arabia was the "most significant" source of funding for Sunni terrorist groups worldwide.
Split into simpler sentences: Relations were hurt over the United States diplomatic cables leak by the whistle-blowing website Wikileaks in December 2010. They suggested that the USA was concerned that Saudi Arabia was the "most significant" source of funding for Sunni terrorist groups worldwide.