At the outbreak of the American Civil War, Johnston was the commander of the U.S. Army Department of the Pacific in California. Like many regular army officers from the South, he was opposed to secession. But he resigned his commission soon after he heard of the secession of his adopted state of Texas. It was accepted by the War Department on May 6, 1861, effective May 3. On April 28 he moved to Los Angeles, the home of his wife's brother John Griffin. Considering staying in California with his wife and five children, Johnston remained there until May. Soon, under suspicion by local Union officials, he evaded arrest and joined the Los Angeles Mounted Rifles as a private, leaving Warner's Ranch May 27. He participated in their trek across the southwestern deserts to Texas, crossing the Colorado River into the Confederate Territory of Arizona on July 4, 1861. Early in the Civil War, Confederate President Jefferson Davis decided that the Confederacy would attempt to hold as much of its territory as possible, and therefore distributed military forces around its borders and coasts. In the summer of 1861, Davis appointed several generals to defend Confederate lines from the Mississippi River east to the Allegheny Mountains. The most sensitive, and in many ways the most crucial areas, along the Mississippi River and in western Tennessee along the Tennessee and the Cumberland rivers were placed under the command of Maj. Gen. Leonidas Polk and Brig. Gen. Gideon J. Pillow. The latter had initially been in command in Tennessee as that State's top general. Their impolitic occupation of Columbus, Kentucky, on September 3, 1861, two days before Johnston arrived in the Confederacy's capital of Richmond, Virginia, after his cross-country journey, drove Kentucky from its stated neutrality. The majority of Kentuckians allied with the Union camp. Polk and Pillow's action gave Union Brig. Gen. Ulysses S. Grant an excuse to take control of the strategically located town of Paducah, Kentucky, without raising the ire of most Kentuckians and the pro-Union majority in the State legislature.

Answer this question "When did Johnston decide to join the confederacy?" by extracting the answer from the text above.
he evaded arrest and joined the Los Angeles Mounted Rifles as a private, leaving Warner's Ranch May 27.