After the Mexican War the Santa Anna dictatorship was overthrown, which began a period known as La Reforma. Liberals were convinced that Mexico had to modernize after the humiliating defeat at the hands of the Americans.
By 1858, La Reforma had devolved into the War of Reform. We can look to our crystal ball to see that the liberals, under Benito Juarez, would win. That, however, would only end with a French invasion and attempt to set up Maximilian, a Hapsburg, as Emperor of Mexico. By that time, however, America would be preoccupied with other problems.
Very recently, a man named Walker, with his Filibuster Army, had been defeated and thrown out of Nicaragua. Walker had seized the country during a time of turmoil, allegedly as one faction's ally, but tried to convert it into an English speaking slave state. Only with great effort did the Central Americans expel Walker and his Filibusterers. Almost as prone to making bad decisions as Buchanan, President Pierce had recognized Walker's regime as the legitimate government.
England still exercised great influence in the region and especially in what is now Belize and the Mosquito Coast of Nicaragua.
So, our Diarist's sour view of American influence in the region vis a vis the Europeans has some grounding in recent events.
I examined the volume of his diary that covers the war. He followed events and wrote about the battles, rumors, news coverage, and a few acquaintances who fought, but I didn't see where he gave his position as pro- or anti-war. He seems not to have considered it from that perspective.