In Costa Rica, the government declared independence from Mexico in October 1822, however, a coup d'état by pro-Mexican monarchists in March 1823 led to the outbreak of a civil war.

Before Filísola could continue to Nicaragua and Costa Rica after his victory in El Salvador, Iturbide was forced to abdicate the Mexican imperial throne and go into exile, and a provisional government was established after the abolition of the monarchy.

On 16 September 1810, criollo priest Miguel Hidalgo y Costilla issued the Cry of Dolores, a call for Mexican independence from the Spanish Empire.

[7] On 24 February 1821, Agustín de Iturbide, a Mexican general fighting for independence, published his Plan of the Three Guarantees in the city of Iguala.

[31][32] Gaínza (who had since assumed the political leadership of both Guatemala and the Consultive Junta),[33] Nicaraguan Bishop Nicolás García Jerez, and the Guatemalan Aycinena clan [es] supported annexation.

[43] "My object is only to manifest to you that the present interest of Mexico and Guatemala is so identical or indivisible that they cannot constitute themselves in separate or independent nations without risking the security of each..." On 28 November 1821, Gaínza received a letter from Iturbide formally requesting the annexation of Central America into the Mexican Empire.

[49] As a result of the annexation, Mexico reached the height of its territorial extent,[1] and the people of Central America were automatically granted Mexican citizenship.

Republican politicians in El Salvador attempted to usurp authority of Central America from Guatemala City and lead a region-wide resistance to Mexican occupation.

[56] Gaínza, who was serving as the captain general of Central America,[57] committed Guatemalan soldiers to support the Mexicans in March 1822 and placed them under the command of Chilean Sergeant José Nicolás de Abós y Padilla [es].

Arzú's army succeeded in occupying San Salvador on 5 April and forced Salvadoran soldiers to abandon the city.

[67][68] During this time, a rumor spread in El Salvador that the United States had sent an expeditionary force of 1,500 soldiers to enforce the annexation, but no such expedition existed.

[69] The envoy arrived in mid-1823, but they were not invited to meet either President James Monroe or Secretary of State John Quincy Adams.

Despite Filísola's numerical advantage, he recognized that attempting to subjugate the rebel army would be difficult as Delgado's symbolic religious support was boosting public morale in San Salvador.

[74] After the rejection, Mexican forces marched on Apopa and Ayutuxtepeque on 7 February 1823, all the while being attacked by Salvadoran soldiers using guerrilla tactics.

[79] The Salvadoran soldiers who fled the city retreated to Honduras under the command of Mariano Prado, where they surrendered to Filísola near the town of Gualcince on 21 February.

[85] When Iturbide abolished the Constituent Congress on 31 October 1822[86] without a new constitution being drafted, some Costa Ricans became frustrated with the Mexican emperor.

On 8 March 1823, the Superior Gubernatorial Junta voted to secede from Mexico, declaring: "The Province of Costa Rica shall be absolutely free and independent of any power, therefore in the use of its rights and the current congress in the exercise of its sovereignty.

[89][90][91] Republican Gregorio José Ramírez was declared as the leader of Costa Rica in opposition of Oreamuno in the city of Alajuela on 1 April.

[93] José Anacleto Ordóñez, a Nicaraguan soldier, merchant, and nationalist who opposed Mexican rule over Nicaragua, launched a rebellion against the pro-Mexican government on 16 January 1823.

Five days later, Ordóñez's rebel forces captured Crisanto Sacasa, the pro-Mexican commander of Granada, and held him as a prisoner of war.

Less than one month later on 25 May, Salvadorans managed to pressure Codallos and the garrison of 500 Mexican and Guatemalan soldiers under his command to leave San Salvador.

Chiapas, Costa Rica, Honduras, and Nicaragua stated that they would boycott the conference until Filísola resigned as captain general and withdrew all Mexican forces from Central America.

The situation in Mexico stabilized after the Mexican Constituent Congress ratified a new constitution in October 1824 and the country became a federal republic.

Iturbide ruled as regent, and then as emperor, from January 1822 until his abdication in March 1823, after which, the three leaders of the provisional government—Bravo, Victoria, and Negrete—served as joint heads of state.

Due to unavailable demographic data at the time, Iturbide reluctantly allowed Central America to have 40 representatives in the Constituent Congress, which he thought was a "prudent" amount.

[136] For Mexico, the annexation of Central America was seen as a way to help stabilize the country's struggling economy, especially the mining and agricultural industries, after a decade of fighting against Spanish rule.

Additionally, leaders in Central America saw annexation as a way to help its own economy by allowing diversification and opening trade to Mexico and potentially Europe.

[142] Sometime between 1823 and 1825, a congressional commission by the government of the Federal Republic of Central America began an investigation into why the mint in Guatemala City had been "reduced" to the "condition of insignificance" it was in.

[143] Additionally, the residents of Guatemala City were forced to raise enough money to pay for the Mexican army's withdrawal from Central America in August 1823.

Slavery was not made illegal until 24 April 1824 by an executive decree and Central America's later adoption of its constitution, however, prior to then, many slaves had already been freed by their owners.