<Tr> <Td> Dec 29 </Td> <Td> The United States admits the Republic of Texas to the Union as the slave State of Texas . The boundaries of the state remain undefined . </Td> </Tr> <Tr> <Td> 1846 </Td> <Td> Apr 25 </Td> <Td> The first skirmish of the Mexican--American War takes place on the Rio Grande near present - day Brownsville, Texas . </Td> </Tr> <Tr> <Td> May 13 </Td> <Td> The United States under President James K. Polk declares war on Mexico, formally commencing the Mexican--American War . </Td> </Tr> <Tr> <Td> Jun 14 </Td> <Td> In the Bear Flag Revolt, American insurgents led by William B. Ide seize the Sonoma Barracks from Mexican officers and declare their intention to found an independent republic in northern Alta California . The so - called "Bear Flag Republic" lasts just 25 days, after which it is subsumed into American military efforts to control California . </Td> </Tr>

When did the cowboy era start and end