The United States celebrated its 250th birthday on Saturday with parades, flyovers and fireworks to come, as President Donald Trump stamped his mark on the landmark anniversary in a deeply divided nation.But weather threatened to spoil Trump's super-sized party in Washington, as tens of thousands who had gathered on the National Mall hours ahead of a campaign rally-style address by the president were ordered to evacuate due to an approaching thunderstorm that followed hours of searing heat.Also read: 'Take India-US partnership to new heights': PM Modi congratulates Trump as US turns 250While many streamed toward exits, chaos broke out as...