By the time war broke out in 1812 Robert Lucas had risen to Brigadier General of the Ohio Militia, and had been tendered appointment in April 1812 as a Captain in the regular army of the United States. A few days afterward there was a call for volunteers, and, instead of waiting for his officer appointment to be formalized, he enlisted as a Private in a volunteer company commanded by his brother. The journal records the campaign from April 25 to August 16, 1812, when Detroit was surrendered. Lucas served as a scout, guide and ranger, and these duties enabled him to be familiar with many facets of the campaign.

Incidentally, twenty years later Lucas was elected Governor of the State of Ohio