WASHINGTON — Army Secretary Dan Driscoll was planning a trip to Kyiv to discuss drone technology with his Ukrainian counterparts when his mission suddenly got more complex. President Donald Trump was upgrading his role, Driscoll was told, to include international diplomat.

The decision has thrust Driscoll to the forefront of the most vexing foreign policy challenge that Trump, by his own admission, has faced since he took office: ending the nearly four-year-long war in Ukraine. It’s a high-stakes foray for Driscoll, a former Army Ranger and financier, that has elevated his profile and fueled speculation inside and outside the Trump administration about where he might land next.

This account of Driscoll’s diplomatic activities and how the administration came to trust him is based on inter

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