Mr. Obama, an Illinois Democrat elected to the United States Senate two years ago, delivered the first major foreign policy address of his Democratic presidential bid to hundreds of supporters in the ballroom of a downtown hotel here. It is the first of several policy speeches he is scheduled to deliver in the coming weeks as he works to define his candidacy with specific proposals an Obama administration would pursue.

“This election offers us the chance to turn the page and open a new chapter in American leadership,” Mr. Obama said. “The disappointment that so many around the world feel toward America right now is only a testament to the high expectations they hold for us. We must meet those expectations again, not because being respected is an end in itself, but because the security of America and the wider world demands it.”

He added: “This is going to require a new spirit, not of bluster and bombast, but of quiet confidence and sober intelligence, a spirit of care and renewed competence.”

In the opening three months of his presidential race, Mr. Obama has solidified his role as one of the leading contenders for the nomination, raising more money than any of his rivals for the primary campaign. But Mr. Obama is also striving to expand his appeal beyond that of a best-selling author and political celebrity as he tackles questions of substance and policy.

Image Senator Barack Obama speaking before the Chicago Council on Global Affairs. Credit... John Gress/Reuters

The United States must build a 21st century military, Mr. Obama said, in addition to “showing wisdom in how we deploy it.” He called for expanding American ground forces, adding 65,000 soldiers to the Army and 27,000 to the Marines. But less than 1 percent of the military can speak Arabic, Mandarin or Korean — a shortcoming he said needs to be corrected through training and recruitment.