Although both wars were fought to preserve the independence of free countries, the conflicts of WWI and the Spanish-American War led to very different consequences. In contrast to the Spanish-American War, WWI led to wide political changes in Europe, the development of new war weapons, and a change in the American social landscape. WWI was also a very expensive war that led to many more casualties than the Spanish-American war.
It has been said that WWI caused the deaths of 9 million combatants and 5 million civilians. It also cost $300 billion in direct and indirect costs, more than any previous war in history. WWI also led to the fall of four autocracies: Russia in 1917, Turkey in 1922, Austria-Hungary in 1918, and Germany also in 1918. Russia sustained great losses during WWI, and it never fully recovered from it. The losses caused Czar Nicholas II to lose his control over the Russian government. Read how the losses Russia sustained during WWI led to the February and Bolshevik Revolutions of 1917.
WWI also led to the fall of the Ottoman Empire (Turkey) in 1922 and affected political changes in the Middle East that has reverberated to modern times. Although Turkey/The Ottoman Empire chose to remain neutral in the beginning, it eventually decided to enter the war on the side of the Central Powers. The Empire sustained insurmountable losses because of the war, and it never recovered its dominance again. After WWI, British forces dominated regions in the Middle East that later became modern-day Iraq, Syria, Lebanon, Jordan, and Israel.
Also, read about the fall of Germany after WWI and how WWI led to the fall of the Habsburg-ruled Austria-Hungarian Empire, resulting in the forming of new nations such as Czechoslovakia and Yugoslavia.
Last, but not least, WWI also ushered in a new technological era and social adjustment in the United States. The exodus of people into cities led to more people participating in the excesses of the Roaring Twenties. Indeed, unlike the Spanish-American war, WWI led to wide-ranging political and social changes that continue to affect our modern age.
In order to get a better answer, it might be helpful to ask a more focused question. There are many aspects of these wars that could be compared and contrasted. Here are a few points that you could make:
- The Spanish-American War was a tiny war that lasted a short time. WWI was a huge war that lasted a long time.
- The Spanish-American War was caused largely by a desire in the US for empire and a desire to prove the "masculinity" of the US in war. People like Theodore Roosevelt thought that the US needed to fight a war to toughen its people and to prove that they were strong. WWI was caused in part by somewhat different factors. It was caused by militarism, nationalism, and competition for power between various European countries. One aspect of this competition for power did involve imperialism, so that aspect of the causes of WWI is similar to a cause of the Spanish-American War.
Outside of the causes, these two wars are rather different because of the huge differences in how big they were.