On April 15, 1861, President Abraham Lincoln called for 50,000 volunteers to fight to preserve the Union. The following day, Bosbyshell joined the Washington Artillerists, a local militia company, which quickly set out for Washington. En route, the company had to march through the streets of Baltimore on April 17. A hostile crowd of Confederate sympathizers gathered; according to accounts in his lifetime, he was struck by a missile variously described as a stone or a brick. Although it gave him a large bruise and momentarily stunned him, the object drew no blood; Private Bosbyshell was purportedly the first man wounded in the Union cause, while an African-American servant, hit a few minutes later by a missile which broke the skin, was deemed the first man to have shed his blood for the Union. However, official records do not list Bosbyshell among the casualties of the Baltimore riots.