Final answer:
Political changes in the late 19th century, such as the rise of imperialism and complex alliances, led to increased tensions between European nations. The unification of Italy and Germany and territorial disputes, especially between France and Germany, set the stage for the onset of World War I.
Step-by-step explanation:
Political changes in the late 19th century played a significant role in heightening tensions among European nations as the 20th century approached. A period of relative peace and stability, known as the Long Peace, was experienced after the Napoleonic Wars due to the deliberate efforts of the great powers to maintain a balance, exemplified by the Concert of Europe's intent to preserve the status quo.
However, as European countries underwent political transformations, the emergence of ideologies like liberalism, conservatism, nationalism, and socialism led to internal and external conflicts. The revolutions of 1848 and subsequent conservative regains of power pointed to underlying destabilization. The unification of nations such as Italy and Germany under conservative leaders who absorbed liberal and nationalistic elements marked the beginning of new nation-states with strong imperialistic ambitions, particularly Germany.
Furthermore, the early 20th century saw the creation of complex alliances that, while originally intended to prevent conflict, actually paved the way for broader military engagements. Imperial striving, such as Germany's expansionist policies and the unresolved Franco-German animosity over Alsace-Lorraine, exacerbated tensions. The burgeoning nationalism, military buildups, and alliance systems all contributed to an environment ripe for conflict, culminating in the outbreak of World War I in 1914.