A typical argument often advanced in favor of the so-called "Reconquista" plan is that the annexation of the southwestern United States was unjust:

The Mexican-American War (1846-1848) was fought primarily to enable the United States to expand at the expense of Mexico. Texas became the focal point of hostilities between an expansionist United States and a recently independent Mexico. Increasingly dominated by white immigrants from the United States, Texas gained independence from Mexico in 1836. This short-lived Texas republic sought U.S. protection against Mexico and possible interference from the British or other European powers.

At the urging of President James K. Polk, Congress approved the annexation of Texas on March 1, 1845. Polk also sent representatives to Mexico to negotiate the purchase of what are today New Mexico and California, but the Mexicans refused and both sides sent troops to the Texas-Mexico border. U.S. forces, led by future President Zachary Taylor, provoked an incident with the Mexican army, and Polk quickly obtained a declaration of war from Congress on May 13, 1846. In August 1847, after 15 months of fighting, the Mexican Government accepted a temporary armistice and began to negotiate a permanent peace settlement with Polk's personal representative, Chief Clerk of the Department of State Nicholas P. Trist. After the Mexican delegation rejected U.S. proposals for a settlement, the armistice was terminated on September 7. The war was won decisively by the United States, culminating with the capture of Mexico City about 1 week later.

The problem is, a few details are being left out. The State Department's paltry history of Texas implies that the whole thing was a quick land-grab and the Texas Republic a short-lived fraud.

Here's another account:

Texas remained an independent republic for almost a decade. Although Texas formally asked to become part of the United States, the American government hesitated. Mexico had made it clear that annexing Texas to the Union would be equivalent to the declaration of war. But on December 29, 1845, President John Tyler signed the bill to admit Texas to the Union as the last act of his administration. Mexico broke relations with the United States.

James K. Polk, who had strongly advocated annexation of Texas and expansionism in general, followed Tyler as President. Polk sent John Slidell, Minister of Mexico, to negotiate, offering to cancel a series of debts if Mexico recognized the Rio Grande as the border between the two countries. Slidell also tried to buy the territories of New Mexico and California.

In turn, Mexico asked the United States to reconsider annexing Texas if it admitted Slidell to negotiations. The United States refused, Mexico declined to talk with Slidell, and Polk ordered troops to the disputed border.

General Zachary Taylor with 4,000 men arrived near Corpus Christi along the Rio Grande in late January 1846. The Mexicans regarded it as an invasion of their territory and threatened to attack if the United States did not remove its troops.

The American troops stayed along the mouth of the Rio Grande, waiting for Mexico to begin hostilities to initiate the war. In April 1846, during a small encounter between American and Mexican troops, several American soldiers were killed. President Polk convened Congress and announced: "American blood has been shed on American soil." The United States officially declared war on Mexico on May 13, 1846.

The war lasted two years with the Mexican Army suffering huge losses. General Zachary Taylor's forces moved south from Texas to capture Monterrey. On February 22, 1847, Taylor's troops marched from Monterrey to Buena Vista and defeated Santa Anna's men, who outnumbered the American force three to one.

The State Department clearly conveys the image of a huge power invading and crushing helpless defenders. Yet the Americans were fighting in a foreign country and outnumbered three to one. Isn't that worth a mention? Isn't it worth a mention that the war began when Mexican General Mariano Arista's troops crossed the Rio Grande into Texas and ambushed a U.S. Patrol? That the first major battle of the War of 1848 took place in Palo Alto, Texas, eight miles north of the Rio Grande?

It seems to me that if the admission of Texas to the United States was legitimate, then the United States was at least obliged to defend it.

Does the State Department feel the same way? Well, how does the State Department feel?

While it's tough to determine the feelings of a bureaucracy, reading the rest of the history they give, the tone seems to be one of remorse:

Although ordered by Polk to return to Washington, Trist remained in Mexico and carried out unauthorized talks with Mexican representatives in late 1847. These meetings formed the basis for the final peace treaty, also negotiated by Trist. Although Polk refused to acknowledge or compensate Trist, he grudgingly accepted the agreement and submitted it to the Senate for ratification. The February 1848 Treaty of Guadalupe Hidalgo (named for a town outside Mexico City) signaled Mexico's surrender and finalized the purchase of New Mexico and California for $15 million.

This conflict had several long-term results: First, it largely completed the continental republic; other than the Gadsden Purchase in 1854, the borders of what would become the lower 48 states were set in 1848. Second, the war spawned a legacy of antagonism between the United States and Mexico that exists even today. Third, through the protests of Henry David Thoreau (of Walden Pond fame) and others, the conflict with Mexico sparked one of the early anti-war movements in the United States. It also helped revive contentious debates over the expansion of slavery into the American West.

Since end of the U.S.-Mexican War, historians have been divided in their interpretations. Some have held the United States cupable. Others blame Mexico. Studies of the literature reveal the majority of writers have taken a balanced view, holding neither country entirely blameless. Despite the fact that there is no hard evidence to support their views, those who blame the U.S. claim that the war was a "shameless land-grab" brought on by the intrigues of President James K. Polk or that it was part of some sinister plot on the part of the so-called "Slavocracy" to extend slavery. These unfounded arguments are nothing new. They are the same ones used by nineteenth century Whig politicians in their attempts to discredit President Polk. The truth is more simple: The war was fought to defend the right of a free people, namely the citizens of the Republic of Texas, to determine their own destiny, that is to join the American union of states. This was a right that the government of Mexico sought to deny them.

Opposition to the war has often been exaggerated. Only a few outspoken Whig politicians, such as John Quincy Adams, were against it. At the time of the war another oft-cited critic, the writer Henry David Thoreau, was virtually unknown outside his hometown of Concord, Massachusetts. Most Americans enthusiastically supported the war. Approximately 75,000 men eagerly enlisted in volunteer regiments raised by the various states. Thousands more enlisted in the regular U.S. Army. There was no need for a draft. In some places, so many men flocked to recruiting stations that large numbers had to be turned away. Thousands of newly-arrived Irish and German immigrants also heeded the call to arms. (Emphasis in original.)

For nearly ten years, Texas was an independent country with disputed borders. Its national sovereignty was given diplomatic recognition by the United States, France, the United Kingdom, the Netherlands, and the Republic of Yucatán. (Interestingly, there was much unrest in Mexico and Yucatán was another breakaway state which resented the dictatorial methods of General Santa Anna, and which actually declared itself neutral during the War of 1848.)

The history of the admission of Texas is complicated and still disputed, but here's the Wikipedia account:

On February 28, 1845, the U.S. Congress passed a bill that would authorize the United States to annex the Republic of Texas and on March 1 U.S. President John Tyler signed the bill. The legislation set the date for annexation for December 29 of the same year. On October 13 of the same year, a majority of voters in the Republic approved a proposed constitution that was later accepted by the US Congress, making Texas a U.S. state on the same day annexation took effect (therefore bypassing a territorial phase). One of the primary motivations for annexation was that the Texas government had incurred huge debts which the United States agreed to assume upon annexation. In 1852, in return for this assumption of debt, a large portion of Texas-claimed territory, now parts of Colorado, Kansas, Oklahoma, New Mexico, and Wyoming, was ceded to the Federal government.

Anyone know who writes history for the State Department?

Is it our official history and are we bound by it?

I mean, I know that government web sites can't be expected to be as democratic as Wikipedia, but shouldn't Texas have the right to weigh in?