Since Russia invaded Ukraine, thousands of Americans and other foreign nationals have signed up to fight for Ukraine, answering a call to action by Ukrainian President Volodymyr Zelenskyy. Now, as the conflict enters its fourth week, a growing number of foreign volunteers are flowing into the capital, signing contracts and receiving weapons and combat training before getting deployed to one of the numerous front lines of the war.They have been compared to the 32,000 foreigners, mostly Americans and Europeans, many of them equally unprepared, who joined the republican forces in Spain’s 1936-39 civil war. That conflict became a losing battle...