American forces included the frigate USS United States and the two sloops-of-war USS Dale and USS Cyane. The squadron arrived in Monterey Bay on October 19 and anchored. Commodore Jones sent his second in command Captain James Armstrong ashore to demand a Mexican surrender by 9:00 am the following morning. The Mexican garrison consisted of only fifty-eight men in an old fort who chose not to resist so when 9:00 am came, fifty American marines and 100 sailors landed and captured the city without incident. It wasn't until the next day that Commodore Jones learned that war had not begun between the United States and Mexico and that the British were not preparing to take control of California. The Mexican troops were freed and then landing party boarded their ships and set sail, saluting the Mexican flag as they exited the harbor. Jones headed for Hawaii, which was just taken over by the British, and he helped restore the Kingdom of Hawaii. The incident proved unpopular in Mexico and in response Monterey's garrison constructed shore batteries and other defenses to guard their city from future attack which came in 1846 during the Mexican War. Thomas Jones was relieved of duty though he eventually served in the war with Mexico.
Prior to the Mexican-American War the Californio forces had routed the Mexican-appointed Governor MIchael Micheltorena and most of his forces from Alta California (Upper California). The Californio Governor Pio Pico, had about 100 poorly armed and poorly equipped soldiers, and was nominally in charge in Alta California; he consolidated his forces in Pueblo de Los Angeles, the then largest city in California with about 1,500 residents.
The main forces available to the United States in California were the about 400-500 bluejacket sailors and US Marines on board the five ships of the Pacific Squadron there. Speculating that war with Mexico over Texas etc. was very possible, the US Navy had sent several additional naval vessels to the Pacific in 1845 and 1846 to protect U.S. interests there and prevent possible British action. It took about 200 days, on average, for ships to travel the over-12,000 miles trip from the East coast around Cape Horn to get to California. More ships assigned to the Pacific Squadron would continue to arrive from 1846 to 1847.
On 26 May, Commodore Sloat received word of the Battles of Palo Alto and Reseca de la Palma, upon which he sent a coded message to Secretary of the Navy George Bancroft that he was leaving for California. Sloat sailed on the frigate Savannah on 8 June, arriving Monterey on 1 July.
They joined the sloop USS Cyane which was already there. There were U.S. fears that the British might try to annex California to satisfy British creditors. The British Pacific Station's ships off California were stronger in ships, guns and men.
On 6 July, Sloat decided to seize Monterey and San Francisco.
Capt. William Mervine of the Cyane came ashore with a small party from the Savannah at 7:30 AM on 7 July, seeking the surrender of the port from the Mexican commandant, Capt. Mariano Silva. Silva replied that he was not authorized to surrender the place. In fact Silva was in command of a nonexistent garrison. The Californio soldiers had already left the town's defenses and gone to Los Angeles. They would have had no gunpowder to use in their few cannons, even if they had stayed.
At 10 AM 225 sailors and marines from Sloat's three warships landed. They formed before the Customs House where Purser Rodman M. Price read Sloat's proclamation of a state of war between the US and Mexico and the annexation of California by the US. The only shots fired were a 21 gun salute to the new U.S. flag fired by each of the U.S. Navy ships in the harbor. The British ships observed but took no action.
Sloat recognized Mexican real estate titles and church lands. He also established justices of the peace when the alcaldes resigned his office.
Capt. John B. Montgomery of the Portsmouth received Sloat's message to seize Yerba Buena (San Francisco) and by 8 AM on 9 July, read the pronouncement at the Customs House. He then replaced the Bear Flag with the American flag. The American flag was run up with a 21-gun salute Montgomery then sent Purser James H. Watmough to notify Fremont at Sonoma and Sutter's Fort
Commodore Robert F. Stockton arrived Monterey Bay aboard the Congress on 15 July and took over command from Sloat. The British ship of the line Colingwood arrived Monterrey on 23 July, and Juno arrived Yerba Buena on 11 July, but both ships did not interfere in the American activities.
The Bear Flag revolutionaries were soon combined with Fremont's 60 man exploratory force to form the U.S. sponsored California Battalin under the command of Fremont. The California Battalion, which varied from 160 to 400 men, drew regular army wages and were used to garrison and maintain order in the towns that had surrendered.
The Americans held northern California but General Jose Maria Castro and Governor Pio Pico planned resistance in the south.