The Red River Campaign, also known as the Red River Expedition, was a major Union offensive campaign in the Trans-Mississippi Theater of the American Civil War which took place from March 10 to May 22, 1864. It was launched through the densely forested gulf coastal plain region between the Red River Valley and central Arkansas towards the end of the war. The offensive was intended to stop Confederate use of the Louisiana port of Shreveport, open an outlet for the sugar and cotton of northern Louisiana, and to split the Confederate lines, allowing the Union to encircle and destroy the Confederate military forces in Louisiana and southern Arkansas. It marked the last major offensive attempted by the Union in the Trans-Mississippi Theater.

The expedition was a Union military operation, fought between approximately 30,000 federal troops under the command of Major-General Nathaniel P. Banks, and Confederate forces under General E. Kirby Smith, whose strength varied from 6,000 to 15,000. The Battle of Mansfield was a major part of the Union offensive campaign, which ended in defeat for General Banks.

The expedition was primarily the plan of Major-General Henry W. Halleck, former General-in-Chief of the Armies of the United States, and a diversion from Lieutenant-General Ulysses S. Grant's plan to surround the main Confederate armies by using Banks's Army of the Gulf to capture Mobile. It was a total failure, characterized by poor planning and mismanagement, in which not a single objective was fully accomplished. Major-General Dick Taylor successfully defended the Red River Valley with a smaller force. However, the decision of his immediate superior, Kirby Smith, to send half of his force north to Arkansas rather than south in pursuit of Banks after the battles of Mansfield and Pleasant Hill, led to bitter enmity between Taylor and Smith.

During the American Civil War, the Union Army, also known as the Federal Army and the Northern Army, referred to the United States Army, the land force that fought to preserve the Union of the collective states. It proved essential to the preservation of the United States as a working, viable republic.

The Union Army was made up of the permanent regular army of the United States, but further fortified, augmented, and strengthened by the many temporary units of dedicated volunteers as well as including those who were drafted in to service as conscripts. To this end, the Union Army fought and ultimately triumphed over the efforts of the Confederate States Army in the American Civil War.

Over the course of the war, 2,128,948 men enlisted in the Union Army, including 178,895 colored troops; 25% of the white men who served were immigrants, and further 25% were first generation Americans Of these soldiers, 596,670 were killed, wounded or went missing. The initial call-up was for just three months, after which many of these men chose to reenlist for an additional three years.