In 1898, the United States entered the Cuban War of Independence.
Though known in Cuba as the Spanish-Cuban-American War, most people know what came after by the name of the Spanish-American War (that name is used in this site to ensure everyone understands).
Great sympathy in the US for Cuba had been building up, but the US Government refused to even recognize the Cuban rebels. The sinking of the USS Maine in Havana harbor furthered war fever.
Many people in other countries saw the war as an unprovoked action of American aggression, but in the US the results were seen as positive. The Cubans, who knew their independence was coming soon, didn't want the US joining in the war. The Spanish knew from the beginning they had no chance. Their navy was no match for the modern American navy, and the war in Cuba had greatly weakened their army. Nevertheless, the Spanish decided to put up a fight. The war began as a battle of the navies.
In Manila Bay and Havana Bay, the American fleets destroyed the old Spanish ships, killing hundreds. Then came a small problem for the United States. Without any recent need for an army, they had few soldiers and those they did have were unprepared. The US army of 28,000 compared with a Spanish one four times that size in Cuba. Quickly the American army began to train and many volunteered. Soon, infantry and cavalry were in Cuba.
The American troops planned to capture San Juan Hill to take Santiago. General Shafter worried about sickness and Spanish reinforcements, and wanted to take it quickly. After taking El Caney, General Lawton was to meet with the cavalry and together they would capture the hill. The greatly outnumbered Spanish troops put up a strong fight at El Caney, killing far more than they lost and delaying Lawton's troops. Though ordered to retreat, Lawton continued fighting, and with the help of the Cuban army, finally won.
At San Juan Hill, 3,000 Americans were ordered to move forward. The cavalry advanced as well, including Roosevelt's famous Rough Riders. Meanwhile, the Cuban troops prevented reinforcements to Santiago by thousands of Spanish soldiers. After some fighting, the Spanish left the hill. They had lost 102 dead to the 223 on the US side. Including wounded, the American force had lost 10% of its soldiers. Large numbers of American troops prepared to attack Santiago, but the Spanish surrendered the city first. In other Spanish colonies, the Americans had little trouble. They took Puerto Rico without resistance, Guam, and the Philippines as well. This war was a disaster for Spain, and marked the emergence of America as a great imperial power.