Meaning of leverage

US energy policy (or the lack thereof) is one driving force behind the end of unipolarity. Since the first oil shocks of the 1970s, US oil consumption has grown by some 20 per cent and, more important, US imports of petroleum products have more than doubled in volume and nearly doubled as a percentage of consumption. This growth in demand for foreign oil has helped drive up the world price from just over $20 a barrel to more than $100 a barrel. The result is an enormous transfer of wealth and leverage to those states with energy reserves.