As he approaches the finish line of the Marine Corps 10K race, Vadym Svyrydenko breaks into a broad smile: He has picked out the blue and yellow of his country’s national flag amongst the red, white, and blue. Mr. Svyrydenko, a Ukrainian, stands out, too. He’s running on “blade” prosthetics, having lost parts of both arms and both legs while fighting Russian-backed separatist forces in 2015.