U.S. forces invade Mexican territory on two main fronts after the declaration of war on May 13, 1846.

The U.S. War Department sends a U.S. Cavalry force under Stephen W. Kearny to invade western Mexico from Jefferson Barracks and Fort Leavenworth, reinforced by a Pacific fleet under John D. Sloat.

This is done primarily because of concerns that Britain might also try to seize the area.

Two more forces, one under John E. Wool and the other under Taylor, are ordered to occupy Mexico as far south as the city of Monterrey.

U.S. Commodore John Drake Sloat, commander of the U.S. Navy's Pacific Squadron, near Mazatlan, Mexico, had received orders to seize San Francisco Bay and blockade California ports when he was positive that war had begun.

Sloat had set sail for Monterey, reaching it on July 1.

Upon hearing of the events in Sonoma and Frémont's involvement, Sloat erroneously believed Frémont to be acting on orders from Washington and orders his forces to occupy Monterey on July 7 and raise the American flag.

Seventy American sailors and marines occupy Yerba Buena on July 9, claiming California for the United States and and raise the American flag.

Later that day in Sonoma, the Bear Flag is lowered and the American flag is raised in its place.

Monterey-based Mexican authority José Castro travels south, apparently to reconcile with his Los Angeles-based rival Pio Pico and resist the U.S. takeover.

Frémont, on Sloat's orders, has brought one hundred and sixty volunteers to Monterey, in addition to the California Battalion.

On July 15, Sloat transferrs his command of the Pacific Squadron to Commodore Robert F. Stockton, who is more militarily aggressive.

He musters the willing members of the California Battalion into military service with Frémont in command.

Stockton orders Frémont to San Diego to prepare to move northward to Los Angeles.

Commodore Robert Stockton replaces Sloat on July 23 and sails down the coast, landing troops led by Frémont at San Diego and putting ashore an additional force near Los Angeles.

John C. Frémont is court-martialed on grounds of mutiny and disobeying orders

On January 16, 1847, Commodore Robert Stockton had appointed Frémont military governor of California following the Treaty of Cahuenga, then left Los Angeles.

Frémonthad  functioned for a few weeks without controversy, but he had little money to administer his duties as governor.

Previously, unknown to Stockton and Frémont, the Navy Department had sent orders for John D. Sloat and his successors to establish military rule over California.

These orders, however, postdated Brigadier FGeneral Stephen W. KearnyKearny's orders to establish military control over California.

Kearny did not have the troop strength to enforce those orders, and was forced to rely on Stockton's Marines and Frémont's California Battalion until army reinforcements arrived.

On February 13, specific orders had been sent from Washington through Commanding General Winfield Scott giving Kearny the authority to be military governor of California.

Kearny, however, had not directly informed Frémont of these orders from Scott.

Kearny had ordered that Frémont's California Battalion be enlisted into the U.S. Army and Frémont bring his battalion archives to Kearny's headquarters in Monterey.

Frémont had delayed obeying these orders, hoping Washington would send instructions for Frémont to be military governor.

Also, the California Battalion had refused to join the U.S. Army.

Frémont had given orders for the California Battalion not to surrender arms, and had ridden to Monterey to talk to Kearny, and told Kearny he would obey orders.

Kearny had sent Col. Richard B. Mason to Los Angeles, who was to succeed Kearny as military governor of California, to inspect troops and give Frémont further orders.

Frémont and Mason, however, were at odds with each other and Frémont had challenged Mason to a duel.

After an arrangement to postpone the duel, Kearny had ridden to Los Angeles and refused Frémont's request to join troops in Mexico.

Ordered to march with Kearny's army back east, Frémont had been arrested on August 22, 1847 when they arrived at Fort Leavenworth and charged with mutiny, disobedience of orders, assumption of powers, and several other military offenses.

Ordered by Kearny to report to the adjutant general in Washington to stand for court-martial, Frémont is found innocent of mutiny, but is convicted on January 31, 1848 of disobedience toward a superior officer and military misconduct.

President James K. Polk, while approving the court's decision, quickly commutes Frémont's sentence of dishonorable discharge and reinstates him into the Army, due to his war services.

Polk feels that Frémont had been guilty of disobeying orders and misconduct, but he does not believe Frémont is guilty of mutiny.

Additionally, Polk wishes to placate Thomas Hart Benton, a powerful Senator and Frémont's father in law, who feels that Frémont is innocent.

Frémont, only gaining a partial pardon from Polk, resigns his commission in protest to settle in California.

Despite the court-martial, Frémont remains popular among the American public.