"Why this American is on the frontlines of Ukraine’s long war" PBS NewsHour 8/29/2019
Answer - Russian expansionism.
SUMMARY: The war between Ukraine and Russian-backed separatists is in its sixth year, with around 13,000 dead and no sign of resolution. The U.S. has provided financial support to Ukraine, but not soldiers. And although there is no U.S. troop presence anywhere near the fight, some American citizens have elected to join the battle on their own. Special correspondent Simon Ostrovsky shares one such story.