I would use that money for a coup to take over the USA. Then I would fix my country up first, and abolish the constitution, senate, courts, and put in their place superior structures based on the governance in Plato's Republic, which is a dictatorship of a philosopher-king and I would be in charge.
Once that happens, I instill a cult of personality and foment jingoistic expansionism among the general population.
Then, declare war and invade Saudi Arabia, take the oil and now take the 500B in yearly revenue from it. The remaining population of it would be genocided and the territory would be annexed as as a state, so also would the remaining gulf states.
Now, the 1 trillion dollar development could begin. I would then have electric drones with dry ice and silver micropartricles seed the atmosphere for rain to begin. I would also invade the whole of North Africa and then divert a section of the Atlantic and or Mediterranean to flood the Sahara, creating an inland sea which would produce evaporation leading to windward-to-leeward rain. There also would be thorium fission reactors and massive pumps from the red sea coast pumping to whatever mountain range is on the left side of the peninsula, creating artificial rivers.
