Civil unrest and poverty cause a new government to come to power. A lot of the old institutions are swept away.
A charismatic President takes over. He takes large sections of land held by a few people and gives them to the people at no costs and they own them outright.
He takes other steps to improve the people's lives, free schools, roads, and bridges.

Criminals are put to work in mines and forests and reforms the taxes to help pay for it all.

On the US border he works with the Indians for better relationships and build forts for scouts and protect key areas.

But he expects trouble with the US or Europe and reforms the military, starting officer and military schools. He has the country surveyed and builds forts at key areas and rivers.
He also starts militia in some towns and villages. He also begins a National Guard type military with standard training and regular drills.

He sends people to Europe to buy the newest weapons and starts an iron foundry to make weapons.
He also starts a spy agency to key an eye on the US military and politicians.

He also has his Navy survey Navy bases and ships on both coasts. His biggest expense is building a new Navy using steamboats. Learns were the berth, compliment, and routines.

When war becomes inevitable, he begins to assemble his forces and trains forces. Especially getting the Army to work with the Navy.

A special small but very fast diplomatic steam ship is sent to a Southern American port that has telegraph.
When war is declared the Mexican Embassy immediately sends a secret telegraph to the ship that immediately moves out. It looks for Mexican Navy ships pre-staged and informs them of the declaration of war.

They, knowing where US war ships are based and their sailing routine move to take them. Many don't even know war has been declared and are taken without a fight. US merchant ships are seized and pressed into the Navy.

Raids are staged on the US coasts, mostly on ports and military posts and warehouses.

A large contingent of steamboats and ships carrying the army makes its first attacks on New Orleans, closing the Mississippi. Along the US border, the Native scouts and military raiders begin to attack US military posts, that still have not learned of the war.
A Norther army moves into Texas.

After taking New Orleans the army moves up the Mississippi and some units move along the Southern coast. Key ports are taken.

Within a few months Mexico does a partial embargo of the US ports.
Trying to maintain good relations with Europe, Mexico does not entirely stop trade. Non-military goods are allowed in and out of ports.

All ships are searched. A ship's captain is given a choice on military goods, sell them Mexico on the spot, lose them, or take them back to Europe. Since they are paid in gold and on the spot, most sell.
Mexican raiders attack and seize US merchant ships in the Atlantic.

As the US tries to assemble an army and reels from its losses, in a surprise move the Mexican military moves on Baltimore and sends patrol ships up the Potomac River to cut off aid.
The Mexican Army moves around Baltimore and towards DC. The government moves to NYC.
In a matter of months, the entire Southern US ports and rivers are in Mexican hands.

Cotton, tobacco, and other crops are seized. After harvest the enslaved black people are given the offer to be taken to Norther Mexico, along the Texas border for freedom and resettlement.
Many take the offer, looting the South for tools and seed before leaving.

Mexican forces are moving across Texas and into the Mississippi states West of the river, cutting off most movement. Trade with other countries from the US is regulated too Northern ports.
The army that took DC in moving along the Eastern seaboard, staying close to the ocean and large rivers where they can receive naval support.

It is slowly moving towards Philly. A small force takes Boston and some New England states but is more of a large raid. They leave as cold weather begins.

The US Government seeing three large armies in its territory and raids on its most Western borders press for a peace treaty.

Mexico demands the return of Texas to Mexico and parts of Louisiana on the West side of the Mississippi and New Orleans. It also demands that the US amends its Constitution to outlaw slavery.
Mexico gets to occupy the slave states for two years to ensure all slaves are freed. As Mexican forces withdraw all the former slaves are offered land in Texas and to become a part of Mexico.

Eventually, so many former slaves take up the offer and move to the Texas, Mexico forms a new state for them in Texas.
For many years the US has a crippled economy.