Oil prices posted their biggest one-day drop in nearly two years Tuesday as a U.S.-led wave of crude has crashed into weak global demand, threatening the stability of some countries and providing an economic lifeline to others. Tuesday’s slide of 4.5% by U.S. crude oil to $81.84 a barrel on the New York Mercantile Exchange left the price down 20% since the start of June. That was the lowest closing price since June 2012, and some analysts predict the price will fall as much as $10 a barrel lower. The same factors that sank prices Tuesday are behind oil’s four-month tumble, which is pressuring countries from Russia to Iran to Venezuela. World-wide demand is stagnant, and the International Energy Agency cut its full-year oil-demand growth forecast Tuesday to the lowest level in five years.