WASHINGTON — A gathering of 50 current and former US officials plus regional and security experts concluded Oct. 14 that the United States should intensify both its diplomatic and military involvement in Syria to degrade and ultimately defeat the group that calls itself the Islamic State (IS).

The third and final phase of a war game, organized by the Brent Scowcroft Center on International Security at the Atlantic Council, examined three scenarios for dealing with IS: stay the course, diplomatic surge and heavy military intervention. The consensus was that the United States should adopt a hybrid of the last two options, including an effort to revive the stalled Geneva process, this time with Iranian participation.

There was also talk of creating a haven for displaced Syrians near the Turkish border, although some participants worried that this would be complicated by Russia’s recent military escalation and Turkey’s anxiety about steps that could strengthen Syrian Kurds.

“We need big-ticket items,” Bilal Saab, a senior fellow at the Scowcroft Center, said Oct. 15 at a public event that disclosed some of the findings of the game.