“If you’re skeptical that this is a problem, consider this: The president of the United States has refused to halt arms sales to Saudi Arabia in part because he is more interested in appeasing U.S. defense contractors than holding the Saudis accountable for the murder of a Washington Post journalist or for the thousands of Yemeni civilians killed by those weapons,” she added.

She ended her remarks by saying, “The world changed in 2016, and it changed in 2018, and I think the world is going to change again in 2020.”

For Ms. Warren, who is considered among the Democratic presidential front-runners should she choose to run, the speech marked the culmination of a yearslong effort to shore up her foreign policy credentials.

She secured a spot on the Senate Armed Forces Committee after the 2016 presidential election, and she has taken high-profile trips to visit troops with Republicans such as Senator Lindsey Graham of South Carolina and Senator John McCain of Arizona, who has since died.

In recent years, progressives like Senator Bernie Sanders were consistently panned by foreign policy experts for a lack of expertise that, they said, amounted to naïveté. Thursday’s speech underscores how proactively Ms. Warren is trying to distance herself from that pack, straddling the line between left-leaning talking points and a serious, informed approach to the world’s dangers.

Among others who have expressed interest in the 2020 Democratic nomination, some have credible claims to foreign policy expertise, including former Vice President Joseph R. Biden Jr. and former Secretary of State John Kerry.

But if Ms. Warren were to announce her candidacy, few of her fellow first-time presidential candidates would have similar foreign policy credentials or have similarly articulated a vision for America’s role in the international order.