Accordingly he sent a force to a Mexican military post known as Sonoma, which with little trouble was taken.
Fremont sent out a couple of messengers to inform the American settlers of what had taken place, but the messengers fell into the hands of General Castro who put both to death.
General Castro sent one of his captains, with quite a force to destroy the Americans, but the officer changed his mind when he found himself in the neighborhood of the detested invaders.

Fremont pursued him for nearly a week, and captured much of his stock and property, but the Mexican was so skilful in retreating that he could not be brought to bay and Fremont returned to Sonoma.
The little force under Fremont now became the rallying point for the American settlers, and before long the Captain had several hundred under his command.

Leaving a garrison at Sonoma, he marched to Sutter's Fort, which was placed under military rule, and then made his way toward Monterey with the purpose of capturing that town.

On his arrival, however, he found the place had already been taken by Commodore Sloat and the American squadron.

The Commodore leaving shortly after, Commodore Stockton succeeded him.
While at Sonoma, Fremont and his comrades had declared the independence of California and adopted the Bear Flag, which was proffered to Commodore Sloat and the Star Spangled Banner hoisted over the camp.
As the Mexican General, Castro, was known to be at Los Angeles, Fremont asked for and obtained a ship on which his force was taken to San Diego.