–lawyer and Civil War general—was born in Johnson County, Indiana. His father moved the family to Illinois when Marcellus was 10 and to Jefferson County, Iowa, five years later. Marcellus secured an appointment to West Point in 1847, but after two years he returned home to care for his widowed mother, five siblings, and the family farm. He studied law and was admitted to the bar in 1851. He began a practice in Lancaster, Keokuk County, but in 1854 he relocated to Des Moines, where he was very successful.
Several years before the Civil War Crocker organized a privately funded militia company, and with Grenville M. Dodge of Council Bluffs he attempted to convince the legislature to create a state militia. After the attack on Fort Sumter, Captain Crocker's company was incorporated into the Second Iowa Volunteer Infantry Regiment on May 27, 1861. Four days later Crocker was elected major, and in September lieutenant colonel. He was promoted to colonel in November and given command of the 13th Iowa Regiment. His early service was in Missouri guarding railroads and other facilities.
In late March 1862 the 13th Iowa joined General Ulysses S. Grant's army in Tennessee. On April 6 Confederate forces surprised and routed Grant's army at Shiloh. Late that afternoon Crocker succeeded his wounded brigade commander. Crocker's reports reflect the panic, but also his firm and steady presence. His regiment suffered 25 percent casualties, but it could have been far worse. His superior noted that Crocker's "coolness and bravery... and disregard to danger... inspired [his troops] to do their duty."?The next day Crocker's depleted brigade supported the successful counterattack. A month later Crocker was given command of the first brigade composed entirely of Iowa troops–the 11th, 13th, 15th, and 16th Regiments. Known thereafter as Crocker's Iowa Brigade, the unit would remain one of the most effective units in the western theater. The unit's mobility later earned it the nickname "Crocker's Greyhounds."
? Crocker was a natural leader, unflappable, and deeply respected by his men. A tough but fair disciplinarian, he drilled his men thoroughly and demanded that his officers lead effectively. An admiring subordinate noted that Crocker had a "passionate temper and is plain spoken" to a fault.
At the Battle of Corinth on October 3, 1862, Confederates again surprised the Union forces. It was left to Crocker's Iowa Brigade to slow down the Confederate advance northwest of town, allowing General William Rosecrans time to organize his forces in Corinth. The well-disciplined Iowans followed complicated maneuvers ordered by calm officers under fire, allowing the Union troops to fight the next day and win. The victory made Crocker a brigadier general.
Owing to a superior's illness, Crocker was put in charge of the Seventh Division of Grant's army as it crossed the Mississippi on April 30, 1863, in its risky maneuver to capture Vicksburg. Crocker's division performed ably at the battles of Raymond and Jackson. At Champion Hill, Mississippi, on May 16, Crocker's men not only plugged a breakthrough on Grant's right, but then sent the Confederates into full retreat. With that action, Confederate General John Pemberton's army was trapped in Vicksburg.
Crocker did not see Pemberton's final surrender on July 4. Weakened from an earlier bout with tuberculosis, Crocker suffered a severe relapse. Grant personally ordered his medical leave. A few months later he was assigned to command troops stationed in Natchez. Crocker participated in General William T. Sherman's campaign to capture Meridian, Mississippi, and then in the early stages of Sherman's Atlanta campaign, but ill health sent him home in May 1864. Upon Grant's recommendation, Crocker was ordered to command a fort in New Mexico to recover his health in a drier climate.
Crocker's political views evolved throughout the war. Before 1861 he was a Democrat, unsympathetic with Southern slave expansionists. As a War Democrat, he supported Lincoln's goal to restore the Union. In 1862 he wrote to Governor Samuel Kirkwood that the war's result had been to put slavery on the road to extinction. In 1863 Crocker declined a possible nomination by Republicans for governor, believing that his contributions lay in the military. A month before his death, Crocker wrote to Governor William Stone that emancipation and black citizenship were useless without suffrage. He strongly urged Iowa Republicans to support that controversial measure.
His health failing rapidly, Crocker was nonetheless ordered to Washington, D.C., in early summer 1865. There he finally succumbed to tuberculosis. He was buried in Woodland Cemetery in Des Moines. Sources Crocker's military records are at the State Historical Society of Iowa, Des Moines. His Civil War career can be traced in The War of the Rebellion: A Compilation of the Official Records of the Union and Confederate Armies (1880–1901). See also A. A. Stuart, Iowa Colonels and Regiments (1865); Grenville M. Dodge, "Gen. G. M. Dodge's Historical Address," Annals of Iowa 4 (1901), 577–94; and Timothy B. Smith, Champion Hill: Decisive Battle for Vicksburg (2006).