Northern lawmakers were hesitant to go to war with mexico because they feared

Northern lawmakers were hesitant to go to war with Mexico because in the North people began to question the system as they increasingly viewed it as immoral and believed that it was wrong for person to enslave another.  By starting a war with Mexico, they believe that the new territories would create more slave states and perpetuate the system further.  Aside from that, President James Polk also wanted to acquire the territories of California, New Mexico and Oregon. Eventually war broke out when American troops to the Rio Grande and Nueces River regard as disputed territories.