Capture of Tucson ( 1846 ) The Capture of Tucson was a United States attack on the Mexican city of Tucson , Sonora , now the present day Tucson , Arizona . The would be combatants were provisional Mexican Army troops and the American Mormon Battalion . Tucson fell in December 1846 without resistance . Capture . The Mexican - American War began after Thornton ' s Defeat in 1846 . This same year the Mormon battalion was dispatched across what they considered the " Great Western Desert " . American forces , of around 500 riflemen and officer s , were commanded by Lieutenant Colonel Philip St . George Cooke . Although there were around 500 men on the Mormon Battalion roster , only an effective force of 360 took part in the trek across the Arizona desert . Eighty - four women and children were also present during the advance across Arizona . Marching northwest to Tucson in November of the said year , the battalion fought their only battle in Arizona , against wild cattle which attacked them near the San Pedro River . After the " " Battle of the Bulls " " , as it is known , the force moved on to the outskirts of Tucson , where they expected to fight the Mexican garrison of Fort Tucson , a former Spanish presidio . The Mexican force consisted of around 200 men , most likely infantry and cavalry plus two small brass cannon s , as well as an unknown force of men from the garrisons of Tubac , Santa Cruz and Fronteras . On December 16 , 1846 , the American enlisted men of the Mormon Battalion arrived at the end of Tucson , to attack the town ' s garrison . The Mexican Captain Antonio Comaduron had received short warning of the approaching Americans . At first he was reluctant to surrender the presidio , but eventually after realizing he was outnumbered , Captain Comaduron decided to withdraw without fighting . He also advised many civilians to abandon Tucson with him . The Mexicans retreated to San Xavier just as the Americans began their assault to take the city . No fighting occurred and a twenty - eight star American flag flew over Tucson for the first time . As soon as colonel ' s army entered Tucson , they began to assure the frightened and staring population of their friendly intentions . Many of the Mormon men were interested in trade . Lieutenant Colonel Cooke ' s soldiers had been low on food so the Mexicans and Pimas bartered meat and bread for cloth , buttons and pins , but only a little food was transferred to the Mormons through trade . Several thousand bushels of grain were left behind by the Mexican garrison . The Mormons took this , and on December 17 they proceeded onto San Xavier . The only shots fired had been from a picket who mistook approaching Mexican civilians for soldiers and fired . Nobody was harmed though . Fifty men under Colonel Cooke were spotted five miles before the mission town , prompting the Mexican Army to retreat again , south towards the Tubac presidio . The Mormons then ended their occupation and continued on their march across the desert . Eventually the Mormons left Tucson and the Mexicans recaptured the city . Tucson , which was said to have 400 to 500 inhabitants at this time , would officially become an American community ten years later in 1856 , after the Gadsden Purchase .