Well, below I'll talk a bit about Santa Anna's motivations, but I want to say off the bat that Santa Anna sent a huge number of his troops along the Gulf Coast, under a trusted subordinate. Two Mexican columns moved east, each outnumbering the Texan settlers. The southern column fought several small battles, but none of them involved the same numbers as the Alamo or had the strategic impact of San Jacinto, so they've largely been forgotten. The presence of this second column was, however, a huge consideration for the Americans and you'll note they moved east instead of attempting to evacuate to the coast.

Santa Anna had a massive numerical advantage over the Americans, but he lacked the strength to occupy the entirety of Texas if it remained hostile. He didn't have the numbers to seal the U.S. border, and even if he had, he didn't want the headache of dealing with thousands of American civilians. In addition, his army was largely composed of raw draftees. He didn't so much mistrust their ability in battle as the likelihood that they would dissolve into the country if they looted the American settlers' possessions after a victory. With much of the country in open rebellion and rivals plotting in Mexico City, Santa Anna needed to return quickly at the head of a disciplined and motivated force.

Much of the campaign went exactly as Santa Anna hoped. A few pitched battles against American garrisons ended with massacres. The terror these massacres spread prompted Sam Houston to order all Americans to retreat east. Santa Anna harassed the retreating Americans, and his cavalry forces almost succeeded in capturing the leaders of the Texan government.

Santa Anna did make some promises of clemency as he moved east, but he had to know they would fall on deaf ears after the massacres at the Alamo and Goliad. He was counting on the vast majority of the Americans leaving and never coming back. As the Americans continued their retreat, though, Santa Anna's plan backfired. The uncompromising brutality of his campaign prompted many settlers to enlist in the army Houston was trying to build, whittling away at Santa Anna's numerical advantage.

Smelling victory, Santa Anna sped up his approach. On the eve of the Battle of San Jacinto, Santa Anna had left behind much of his army, and he was facing a much larger and more disciplined force than he had anticipated. Santa Anna's forces had been marched to the point of total exhaustion. The rest is history.