The war with Mexico in 1848 was simply an aggressive, land-grabbing, politicians' war, and will always be a blot on the Christianity of the United States. The lands of which Mexico was robbed were of course of great material value to the United States, but the less said about the justness of their acquisition the better.
The question of the Civil War is complicated by the moral issue of the abolition of slavery, underlying the political issue of the right of the South to secede, which was the ostensible cause of the war. Slavery in the United States was an evil, both ethically and economically, and, as its abolition was a result of the war, that war is justifiable both in the court of Jesus and in that of Nature. But that result was an incident of the war, and the North would never have taken up arms on the simple issue of forcing the abolition of slavery on the South.
On the ostensible, political issue which started the war--the right of the South to secede from the Union--there is room for much difference of opinion. Despite the technical and labored constitutional arguments of Von Holst and others, it is rather difficult to understand why, if the South believed that its happiness and prosperity were being imperiled by a further continuance of its union with the North, it had not the same right to break that union in 1861 as the colonies had the right to break their union with England in 1776. At the outbreak of the war, there were many in the North, beside Horace Greeley and Vallandingham, who thought it morally wrong to compel the South by force to remain in a union that had become hateful to it.
A great writer on American history (Sumner) has said that whenever some geographical section of our country becomes saturated with the idea that its material interests are being sacrificed to the interests of other parts of the country, and it sees no hope of redress, it will begin to talk secession. It was true of New England at the time of the Hartford Convention. It was true of the South in 1820, 1831 and 1860. It was true of the Pacific States shortly after the Civil War, when they feared that Congress would not pass their desired Chinese Exclusion Acts.
It would be difficult to justify the Spanish War of 1898, in the court of Jesus. It was mostly the work of the newspapers and politicians. Nine-tenths of the people of the United States were ignorant of suffering great grievances from Spain, until the Jingo journals demonstrated the fact to them. It is safe to say that this war would never have occurred if Spain had been a great naval power like Germany or Great Britain. The same assertion may be made of the war against Mexico in 1848. In studying the Jingo spirit which encourages wars, it will usually be found that the strength of this spirit varies in the inverse ratio to the supposed war-strength of the other party to the fight. Nations are much like school boys in this respect. It is quite probable that the war of 1812 would not have been brought on, except for the mistaken idea of Henry Clay and his hot-headed followers from the West that the United States could easily overrun Canada, and dictate peace to England in Halifax.