Bahrain. Lees had already noted that, in such an eventuality, Qatar should be investigated again. After lengthy negotiations on 17 May 1935, Abdullah bin Jassim signed a concession agreement with Anglo-Persian representatives for a period of 75 years in return for 400,000 rupees on signature and 150,000 rupees per annum with royalties. As part of the agreement, Great Britain made more specific promises of assistance than they had in earlier treaties. Anglo-Persian transferred the concession to the IPC subsidiary company Petroleum Development (Qatar) Ltd. in order to meet its obligations under the Red Line Agreement. Bahrain claimed rule over a group of islands encompassing the two countries in 1936. The largest island was Hawar Islands, situated off the west coast of Qatar, where the Bahrainis had established a small military garrison. Britain accepted the Bahraini claim over Abdullah bin Jassim's objections, in large part because the Bahraini sheikh's personal British adviser was able to phrase their case in a legal manner familiar to British officials. In 1937, the Bahrainis again laid claim to the deserted town of Zubarah after being involved in a dispute involving the Al Nuaim tribe. Abdullah bin Jassim sent a large, heavily armed force and succeeded in defeating the Al Nuaim. The British political resident in Bahrain supported Qatar's claim and warned Hamad ibn Isa Al Khalifa, the ruler of Bahrain, not to intervene militarily. Indignant over the loss of Zubarah, Hamad ibn Isa imposed a crushing embargo on trade and travel to Qatar. Drilling of the first oil well began in Dukhan in October 1938 and over a year later, the well struck oil in the Upper Jurassic limestone. Unlike the Bahraini strike, this was similar to Saudi Arabia's Dammam field discovered three years before. Production was halted between 1942 and 1947 because of World War II and its aftermath. The disruption of food supplies caused by the war prolonged a period of economic hardship in Qatar which began in the 1920s with the collapse of the pearl trade and was exacerbated in the early 1930s with the onsets of the Great Depression and the Bahraini embargo. As was the case in previous times of privation, entire families and tribes moved to other parts of the Persian Gulf, leaving many Qatari villages deserted. Abdullah bin Jassim went into debt and groomed his favored second son, Hamad bin Abdullah Al Thani, to be his successor in