Ryan Wesley Routh, the 58-year-old man who was arrested on Sunday in connection with what the F.B.I. described as an attempted assassination on former President Donald J. Trump, had expressed the desire to fight and die in Ukraine. Mr. Routh’s posts on the social media site X revealed a penchant for violent rhetoric in the weeks after Russia’s invasion of Ukraine in 2022. “I AM WILLING TO FLY TO KRAKOW AND GO TO THE BORDER OF UKRAINE TO VOLUNTEER AND FIGHT AND DIE,” he wrote. On the...