Conditions had been steadily worsening along the Rio Grande. The United States claimed the Rio Grande as the international border while the Mexican Government claimed the Nueces was the proper border. Early in 1846, General Zachary Taylor built a fort on the Rio Grande opposite the Mexican town of Matamoros. In April, the Mexicans countered by sending a force of about 1600 cavalrymen across the Rio Grande where, on April 25, they overwhelmed a force of 60 dragoons under U.S. Captain S. B. Thornton. Mexican forces at Matamoros steadily grew stronger in April. By the end of the month, General Taylor had become concerned about his lines of communication with his lightly held main base at Point Isabel, near the mouth of the Rio Grande. Therefore, on May 1, Taylor moved the bulk of his army to Point Isabel, leaving a small detachment of artillery and infantry under Maj. Jacob Brown at the fort opposite Matamoros. The Mexicans soon placed this fort (later named Fort Brown) under heavy attack. On May 7, Taylor moved to the rescue with about 2,300 men. On the morning of May 8, when little more than half way to the fort, the Americans came face to face with the enemy, a force numbering perhaps as many as 6,000 men, commanded by General Mariano Arista. Its right flank rested on an elevation known as Palo Alto (after which the engagement was named). Taylor moved unhesitatingly into battle, using his artillery to cover the deployment of the infantry. The engagement continued until nightfall, when the Mexicans withdrew. Effective use of artillery fire was largely responsible for the American victory. American losses were 9 killed and 47 wounded. The Mexicans suffered more than 700 casualties, including about 320 deaths.