Foreign fighters in Ukraine speak out on their willingness to serve: 'I had to go':

When Andy Huynh watched the news of Russia's invasion of Ukraine in February, he started losing sleep. All he could think about was the struggle of the Ukrainian people against an aggressor he felt was violating their sovereignty and opening the world up to a third World War.

"All my personal problems didn't feel important anymore … It felt wrong just to sit back and do nothing," he said. "I had to go."

The Alabama man was not alone. Two days after the invasion, Ukraine's President Volodymyr Zelenskyy called for "friends of Ukraine, freedom and democracy" to serve as volunteers in the Ukrainian military. More than 20,000 volunteers from 52 countries responded, many of whom had served in the U.S. Army, British Army, and, like Huynh, the U.S. Marine Corps, according to Ukrainian officials.

Their experience is credited by Zelenskyy for bolstering the war effort for Ukraine, especially since NATO countries have rejected sending ground troops in fears of starting their own conflict with Russia. Russian President Vladimir Putin announced in March that 16,000 volunteers from the Middle East would be joining his country's fight.

Tanya Mehra, a senior research fellow at the International Centre for Counterterrorism at The Hague, said the mobilization of foreign fighters on battlefields dates to 1816 and they have played prominent roles in conflicts in Afghanistan, Bosnia, Chechnya and Somalia since the 1980s.

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The evolution of foreign-born fighters has created distinct classes of fighters, from mercenaries who join conflicts primarily for financial gain, Mehra said, and others who are driven by ideological reasons. Mercenaries, she said, who are outsourced contractors for small governments, tend to be associated to "increases in violence and higher civilian casualties," which can prolong the conflict, whereas foreign fighters become part of the state military, which makes them "accountable for the acts they have committed."

Many of those foreign fighters serving in Ukraine tend to be older than your average soldier, and in a stage in their lives where they felt they could help through their years of experience.