Less than one month before Mike Johnson’s sudden ascent to the speakership, the Louisiana Republican, then a little-known member, joined with most GOP lawmakers to vote against $300 million in U.S. security assistance for Ukraine. Now, as speaker, Johnson has surprised many on Capitol Hill by publicly and repeatedly calling Ukraine aid a critical priority for the House. Where Johnson stands on Ukraine and how hard he plans to push for tens of billions of dollars in new aid could shape the future of the embattled country, just as the U.S. is facing questions about its commitment to Kyiv and...