Montana Territory

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The reorganization of the Idaho Territory in 1864, showing the newly-created Montana Territory.

The Montana Territory was an organized territory of the United States that existed between 1864 and 1889.

The territory was organized out of the existing Idaho Territory by Act of Congress and signed into law by President Abraham Lincoln on May 28, 1864. The areas east of the continental divide had been previously part of the Nebraska and Dakota territories and had been acquired by the United States in the Louisiana Purchase.

The territory also included a portion of the Idaho Territory west of the continental divide and east of the Bitteroot Range, which had been acquired by the United States in the Oregon Treaty, and originally included in the Oregon Territory. (The part of the Oregon Territory that became part of Montana had been split off as part of the Washington Territory.)

The boundary between the Washington and Dakota territories was originally described as the continental divide (shown on the first map). However, at that time the divide had not been formally surveyed. The original intent was to divide the Montana Territory off the Idaho Territory along the same line. However, when the formal survey was done, the survey team followed the wrong mountain ridge and departed from the continental divide at the point shown in the third map. This resulted in the Montana Territory being larger, and the Idaho Territory smaller, than was originally intended. This explanation may be an urban legend though, and there appear to be sources stating that the division along the Bitteroot range instead of the continental divide is exactly what was proposed.

The boundaries of the territory did not change during its existence. It was admitted to the Union as the State of Montana on November 8, 1889.