It's the most exciting US election campaign for decades but, as a historian, I'm also struck by deeper currents under the surface of events.

My new series for BBC Radio 4 America, Empire of Liberty takes as its title Thomas Jefferson's prediction in the 1800s that the United States would be a great"empire of liberty".

Empire and liberty have been recurrent and often conflicting themes in America's development and I think they're also reflected in this year's rival presidential candidates.

John McCain embodies the martial, imperial America.

The son and grandson of admirals, he was a career navy pilot who nearly lost his life in Vietnam and spent more than five years there as a prisoner of war, enduring brutal torture.

Mr McCain is an ardent patriot, convinced of America's mission in the world and a strenuous supporter of the war in Iraq.

His story reminds us that the United States is a country made by war.

In the 19th Century, the struggle with Mexico in 1846-1848 won the West, and 620,000 Americans died in the appalling Civil War of 1861-1865.

And a succession of 20th-Century wars made and sustained the US as a global superpower, from Pearl Harbor in 1941 to Kuwait half a century later.

Given the number of Americans who have served in these conflicts, it is not surprising that veterans have become one of the most influential lobby groups in American politics.

Original sin

Barack Obama couldn't be more different from Mr McCain.

His election to the US Senate in 2004 made him only the fifth African-American Senator in history.

In 2008 he became the first African American to be nominated as a major party's candidate for the presidency.

These statistics remind us that the United States is still striving to make good its resonant promises about being the land of liberty.