He said that 237 years ago American colonists declared independence from Britain and waged a revolution to achieve it, a bold act he called "nearly unthinkable," and that few thought they would prevail.
"But for the first of many times to come, America proved the doubters wrong," Obama said in a weekly radio address.
America was now a "global defender of peace and freedom," he added.
Of people in the military, he said they had defended the country at home and abroad and fought on the nation's behalf to make the world a better place.
"People in scattered corners of the world are living in peace today, free to write their own futures, because of you," the president said.
"We are grateful for your service and your sacrifice, especially those still serving in harm's way, and (for) your families here at home."
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