US Army Sergeant Bowe Bergdahl was the only remaining US soldier captured in recent conflicts in Iraq and Afghanistan, officials said.
Bergdahl, who was held for nearly five years by the Taliban, has been released and is now in US custody.
"It is our ethos that we never leave a fallen comrade. Today we have back in our ranks the only remaining captured soldier from our conflicts in Iraq and Afghanistan. Welcome home SGT Bowe Bergdahl," US Army General Martin E. Dempsey said in a statement.
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Mladic, now 75, was the military commander of Bosnian Serb forces in the 1990s, who fought against the Bosnian Croat and Bosniak armies.
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His representatives said Cassidy, who was hospitalised last week, died "surrounded by those he loved."