Mexico attacked Texas?

You're begging the question.

One main reason Anglos in Northern Mexico rebelled against Mexico to secede and establish Texas as an independent state was slavery. The Mexicans, who were abolitionists, had just cause to attack them, unless you're one of the sham 'patriots' who support the Confederacy over the Union. The Lone Star Republic allowed slavery, when Mexico didn't, and when it became part of the US, it was a slave state that rebelled against the US to form the Confederate slavocracy. From Wikipedia:

'The first Texas provisional government was formed at San Felipe de Austin on November 7, 1835. This council passed a declaration of support for the 1824 Mexican constitution, and appointed a governor and other officials, though it stopped short of declaring Texan independence. The first declaration of independence for modern Texas, by both Anglo-Texian settlers and local Tejanos, was signed in Goliad on December 20, 1835. The Convention of 1836 was convened at Washington-on-the-Brazos with Richard Ellis presiding, and the Texas Declaration of Independence was enacted on March 2, 1836, effectively creating the Republic of Texas.

'Four days later, the thirteen-day Siege of the Alamo ended as Mexican General Antonio López de Santa Anna's forces defeated the Alamo's approximately 183 defenders (the estimate of 183 is disputed, as a number of people appear to have been excluded from the list; experts say it is likely that the defenders from many U.S. states together with those of direct Mexican descent totaled over 200[citation needed]). The Alamo was outside the then-sleepy town that would eventually become the center of the city of San Antonio. Remember the Alamo! became the battle cry of the Texas Revolution that most remember, but in fact this was a shortened version of the actual cry, which was "Remember the Alamo, Remember Goliad". At Goliad, Santa Anna had Colonel James Fannin and 341 of his men—who had surrendered at the Battle of Coleto—marched one mile out of town and massacred.

'The Battle of San Jacinto was fought on April 21, 1836, near the present-day city of Houston. General Santa Anna commanded a force of 1,600 men, of which more than 600 were killed and the rest captured by Texas General Sam Houston's army of 800 Texians, while only nine Texians died. Santa Anna was captured the next day dressed as a poor Mexican peasant. During this battle Sam Houston was wounded in the leg, but would recover. Houston also lost control of his men as anger over the Alamo and Goliad overcame his troops; they showed no mercy for those 18 minutes it took to destroy Santa Anna's army. Houston, however, spared Santa Anna's life, and forced him to sign letters to his two remaining armies instructing them to leave Texas. This decisive battle resulted in Texas's independence from Mexico. With a population of 30,000 Anglo-American Texians, 5,000 African-Americans (most of them slaves; something which had been a point of contention as slavery had already been fully abolished under Mexican law) and 3,470 Hispanic Tejanos, this was quite an accomplishment even with the approximately 14,200 Native Americans, mostly Comanche, staying out of the war....

'On February 28, 1845, the U.S. Congress passed a bill that would authorize the United States to annex the Republic of Texas. 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 specifically endorsed slavery and the slave trade. This constitution was later accepted by the U.S. Congress, making Texas a U.S. state on the same day annexation took effect, December 29, 1845 (therefore bypassing a territorial phase)[1]. 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.'

You must have watched too many Davy Crockett movies as a kid.

Now, what was your point about a 'lawful' nation?