Most interesting U.S. war (1861-1940)

I'm defining U.S. involvement loosly.

This one basically dissolved the last remnants of the Spanish Empire in the New World and, some would argue, was the starting point of the new American Empire.

It was primarily to free Cuba from Spanish control, but it had the result of bringing Puerto Rico, Guam, the Philippines and Cuba (under the Platt Amendment) under U.S. control. It dramatically altered the relationship of the U.S. not only to Central America, but also to the Far East.
The Spanish Civil War (1936-39) was just that, a civil war. Foreign powers were certainly involved to the extent that they took sides between the Loyalist forces and the military under Gen. Francisco Franco. The Germans tried out their new Blitzkrieg style of warfare in support of Franco. And many Americans volunteered to support the Loyalists, but it had nothing to do with the Spanish-American War.

After the Mexican war. War fever was built up in the yellow press with stories of Spanish atrocities in Cuba. Aside from Cuba, gaining a foothold in Asian markets by taking over Spanish colonies and concessions was the unspoken goal.

It also got us into our first experience fighting against an insurgency, in the Phillipines. Victory over the insurgents was declared several times over the next 40 odd years, but they were never fully defeated.

Most definitely the most interesting as the ramifications lasted far longer than the actual hostilities.

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lasted far longer than open hostilities. The war really re-defined the role of the U.S. as a hemispheric power, and, with our access to Asian markets more fully assured, it laid the groundwork for the U.S. to assume a role as a player in the global power struggle with the old European colonial powers.
The causes of the war were pretty interesting as well. Everyone knows about the explosion and sinking of the USS Maine in Havana harbor as the actual spark for hostilities. But the U.S. had been jostling for the removal of Spanish dominance in the region for some time previous to that event. The insurgency in Cuba in the late 1890's against Spanish rule was seen as a destabilizing influence in the Caribbean that could undermine the prospect of building a cross-isthmus canal in Central America. The canal had been a concept for decades. The French had actually formed a company and attempted the project in the 1880's but had been defeated by malaria and engineering problems.
The formation of Panama as an independent country (originally a part of Columbia), the original proposed route of the Canal (across Nicaragua), the medical challenges (yellow fever and malaria)overcome and the monumental engineering reality of the canal could each be a separate thread for discussion.