Mr. Webb, who prides himself on his Scotch-Irish ancestry, has long been something of a renegade, a persona that vividly manifested itself after Sept. 11, 2001, when he began denouncing what he saw as the transformation of the American presidency into a European-style monarchy that could capriciously pursue wars whenever and wherever it chose. Unlike Mrs. Clinton, who continues to struggle to explain her vote for the Iraq war, Mr. Webb publicly attacked the George W. Bush administration in 2002, presciently asking, “Do we really want to occupy Iraq for the next 30 years?” As a member of the Armed Services and Foreign Relations Committees he also castigated the Obama administration for its intervention in Libya in 2011. He was right. It’s a move that has boomeranged, creating further instability and emboldening jihadists across the region.

During and after the Libya intervention, Mr. Webb made it clear that he believed American democracy was imperiled by the failure of Congress to question the judgment of military leaders and the president. He has put his finger on a problem that academics like Tufts University’s Michael J. Glennon, the author of “National Security and Double Government,” see as a product of an entrenched national security bureaucracy that essentially performs an end-run around Congress and even reform-minded presidents.

In contrast to Mrs. Clinton, who has gotten into hot water for trying to retroactively amend her views and record, Mr. Webb did not arrive at these beliefs casually or opportunistically. As his recent memoir, “I Heard My Country Calling,” makes clear, his opposition to ventures abroad is as much viscerally emotional as intellectual. Growing up as a self-described military brat, he spent his formative years in Britain, where he saw firsthand the effects of loss of empire and the devastation wrought by World War II. “Britain was bled out and spent out,” he writes. “They understood the great price of the recent wars in a much more sobering way than did most Americans.”

After he returned from war-torn Beirut just before a truck suicide bomber destroyed the Marine Corps headquarters in October 1983, he felt a nagging irritation as he rode home in a taxi early in the morning along George Washington Memorial Parkway. Then he realized that the calm silence was bothering him; it was both the emblem of America and the “protective vacuum that surrounds our understanding when it comes to the viciousness that war brings to so many innocent noncombatants in other lands.” Mr. Webb’s exposure to foreign societies gave him the ability, much like President Obama, to view America as both an insider and an outsider.

Whether Mr. Webb will attempt to begin a successful maverick campaign is an open question. But he is an eloquent and forthright speaker whose foreign policy experience would make it difficult for Mrs. Clinton to paint him as an isolationist or a novice who will leave America open to attack, as she attempted to do to Mr. Obama during the 2008 primaries. On the contrary, it’s Mrs. Clinton whose interventionist foreign policy record leaves her politically vulnerable.