One of the main arguments the book I read on McKinley made was that he, and not Theodore Roosevelt was the first modern president.
One of the main things the author does to back up this claim is to illustrate how McKinley, like modern presidents, heavily relied on the military to carry out the government's foreign policy objectives.
William McKinley certainly did rely on the scaled up armed forces to carry out his political goals, but I think that rather than emphasizing military adventurism as a "modern" phenomenon, it's more important to realize that at this point in our nation's history, we turned away from the Monroe Doctrine and started to more willingly enter into entangling alliances.
He intervened in Cuba, which has parallels to the Vietnam conflict. The Spanish American War even had it's own Gulf of Tonkin type causus belli with the sinking of the battleship Maine off the coast of Cuba. Most accounts seem to hold that the ship blew up because of an accident, not a Spanish attack, but this did nothing to dissuade the pro war newspapers from describing this as another act of Spanish aggression. Ultimately, the United States swept through Cuba and secured a treaty within three months, winning control not only of Cuba, but also Puerto Rico, Guam and the Philippines.
The war up to this point had been very successful and helped bring McKinley's successor, Theodore Roosevelt even more national fame when he left his post as Secretary of Navy to lead the "Rough Rider" regiment to victory in Cuba.
The problems came when the United States took control of the Philippines, liberating the residents from Spanish control, but facing a homegrown insurgency not long after the Spanish left. We don't read about the Philippine-American war very often in our history books, but it was one of the largest issues of the late 1800's and early twentieth century. The United States army was stuck there for over a decade, fighting a homegrown insurgency made up of religious fanatics (sound familiar?). Estimates of civilian and military casualties are all very political and imperfect, but some estimates say as many as 35,000 Fillipino soldiers died and as many as 200,000 civilians died either directly from the war or indirectly from actions the Americans took. In addition, torture such as the "water cure" (an early form of waterboarding) is believed to have been commonplace along with scorched earth policies that allowed the burning down of entire villages.
The story was the same then as it is now. The United States could have held the high ground after they reached a strategic goal and the Spanish withdrew from Cuba. But just as we feel we need to bring democracy to the middle east now, Americans back then thought along the same lines and wanted to completely topple the Spanish empire. The cost in blood and treasure and moral authority however, was much more than the United States paid the dying Spanish empire to acquire the Philippines.