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Now the Portsmouth Square, this was the site of the raising of the American flag after Yerba Buena (now San Francisco) was captured during the Mexico-American War. During the war, Captain John Berrien Montgomery of the USS Portsmouth was ordered to seize Yerba Buena. On July 9, 1846, the first American flag was raised near the Mexican adobe custom house in the plaza that would eventually be named Portsmouth Square in honor of the ship.

  • Scene from Dirty Harry filmed by Portsmouth Square Garage. Scorpio played by Andrew Robinson is leaving the garage and entering the Square
  • Portsmouth Square today
  • Portsmouth Square in 1851
  • Marker with replica of first public school in California
  • Dirty Harry movie poster
  • Though inaccurate, this sketching made soon after the battle, shows the flag raising in Yerba Buena
  • Monument on site of the flag raising
Upon declaring war on Mexico on May 13, 1846, President James K. Polk immediately called for the capture of California from the Mexicans. California had been a territory that Polk had desired ever since the annexation of Texas in 1845, and when war with Mexico broke out, he saw the perfect opportunity to take hold of the land he wanted. The US Pacific Squadron was given the order to occupy every important port and city in California, with force if necessary.

On July 7, 1846, the ships USS Savannah, Cyane and Levant
 captured the Alta (Upper) Californian capital city of Monterey without firing a shot. This procedure of occupation would set the stage for the Battle of Yerba Buena, which would follow a few days later.

On July 9, 1846, the USS Portsmouth, captained by Commander John B. Montgomery, sailed into San Francisco Bay, with the intent of capturing the town of Yerba Buena. Aboard the Portsmouth were 220 sailors and enlisted men, along with a contingent of 27 marines. The small Mexican force garrisoning the Presidio did not fire upon the USS Portsmouth, for fear that they would become obliterated. The Portsmouth landed a distance off the shore, and a group of sailors, soldiers and marines, as well as Montgomery and his staff, disembarked in rowboats for Yerba Buena.

Upon landing on the shore, Mexican soldiers held their fire, and the Californios grouped together to watch the American force. Montgomery and his force walked up to the flagpole in the town square, where the Mexican flag was flown. He quickly tore it down, and hoisted the Stars and Stripes in its place, proclaiming that the town of Yerba Buena, and all of the land surrounding it, belonged to the United States.

After Montgomery's speech, the marine band began to play Yankee Doodle, and the USS Portsmouth fired a 21 gun salute, to celebrate the capture of Yerba Buena. Following the capture of the town itself, Montgomery ordered a detachment of troops to seize the Presidio of San Francisco, and confiscate any weaponry they found, which the detachment did without conflict. Thus, the pueblo of Yerba Buena, and the land that would become eventually known as San Francisco, was captured by the United States of America.

The site was also home to the first public school built in California in 1847. Many other events occurred in the same area, since renamed Portsmouth Square in honor the USS Portsmouth

On May 11, 1848, the discovery of gold was announced when Sam Brennan showed his gold to a crowd. On June 12, 1849, a crowd was gathered at the plaza, demanding election of delegates at the Monterey Constitutional Convention. An assembly was organized on July 16, 1849 to fight against a lawless body 'The Hounds.' On August 29, 1850, a memorial service was held after the death of President Zachary Taylor, also a war hero of the Mexican War. The First Admission Day celebration was held October 29, 1850 when California became the 31st state of the United States. On June 1, 1852, a crowd protested against the purchase of the Jenny Lind Theatre as the city hall. On September 18, 1859, Colonel E.D. Baker delivered an oration after U.S. Senator David C. Broderick was killed in a duel with California Chief Justice David S. Terry.

Director Don Siegal filmed a scene from the 1971 movie Dirty Harry in the Square. As the character "Dirty Harry" follows "Scorpio" it is possible to see the stone bridge joining the park to the Chinese Culture Center of San Francisco, at the Hilton Financial.

In 1987, the park underwent its second major renovation. The first phase involved installing new elevators and bathrooms on the top of the park. The second phase began in 1994, included installation of child play structures, Chinese Chess tables, benches, and landscaping. Phase three included the construction of a new community room and play areas. This $3.9 million renovation was completed and the park was opened to the public in 2001

Hoover, Mildred Brooke; Douglas E. Kyle (2002). Historic Spots in California. Stanford University Press. pp. 353–355. Portsmouth Square: The Plaza. Found SF. Accessed March 26, 2017.