 Hamilton akasarudza kutevera chinangwa cheUnion uye akagadzwa colonel we3rd Wisconsin Infantry musi waMay 11, 1861, uye akasimudzirwa kuva brigadier general musi waMay 17. Akabva apiwa basa rokutungamirira brigade muMajor General Nathaniel P. Banks division yeArmy of the Potomac. Pakutanga kwePeninsula Campaign, Hamilton akapiwa kutungamira kwe3rd Division, III Corps. Akatungamirira boka iri panguva yeKukombwa kweYorktown, asi akawana kutsamwa kwaGeorge B. McClellan nokuda kwekusateerera. Achiregeredza chokwadi chokuti kuregedza Hamilton kwaizogadzira vavengi vane simba vezvematongerwe enyika, McClellan akabvisa Hamilton musi waApril 30, 1862 pasina tsanangudzo. Apo Lincoln akanongedzera kukusafanira kwechiito chaMcClellan, McClellan akataura kuti Hamugoni kuita chinhu chipi nechipi chakarongwa kukuvadza hondo yangu... pane kudzorera Gen. Hamilton kudhivhisi yake. Kugadzirira kwekukomba kwemwedzi kusati kwapera, Hamilton akaendeswa kuWestern Theater kwaakatungamira Chikamu chechitatu muuto reMississippi muhondo dzeIuka neKorinde. Achitevera Korinde muna Gumiguru 1862, akasimudzirwa kuva mukuru mukuru, anoshanda kubva munaGunyana 19, 1862. 