There are no fundamental differences between now and what international politics used to be in the first half of the 20th Century. It is true that the post-WWII period has been more peaceful, but it is not because of a fundamental transformation in the way international politics works.
To state that there are no fundamental differences between international politics in 1900-45 and afterwards would be to carry the argument to an extreme, even though the continuities are greater than the discontinuities. Above all else, the liberal, democratic states and empires in the U.S. And Western Europe were highly interventionist and aggressive in the developing world and Global South long before World War II, and this did not change in the Cold War and post-Cold War eras. Even governments that were democratically elected were sometimes overthrown and replaced by more pliable regimes, such as the 'friendly' dictators of Central America and the Caribbean. At the same time, though, there has also been far more harmony and cooperation between the Great Powers since 1945 than in the previous fifty years, especially through NATO and the European Union. America's alliance with Japan, Britain, France and Germany has survived various stresses and strains over the decades, and even the collapse of the Soviet Union, and this requires an explanation. None of the imperial powers has fought a major war since the invention of nuclear weapons, even though they have intervened frequently against the non-nuclear states of the developing world. Perhaps this alliance is explained by political and ideological affinities, as liberals maintain, or by cultural affinities as opposed to Muslim and Orthodox civilizations, as Samuel Huntington explains -- although admittedly Japan is left as quite an outlier here. Realists would counter that self-interest holds the liberal powers together, particularly against political, military and economic threats they face from outsiders, and in reality they do seem to operate on the basis of mutual self-interest between capitalist powers, with the U.S. still playing the role of hegemon, despite its weakening condition in recent years.
Although the U.S. was not a global superpower before the Second World War, there have been a number of fundamental continuities between the pre- and post-1945 periods, especially in relations with Latin America and other countries in the Global South. James K. Polk planned a war of aggression against Mexico and provoked an incident on the Rio Grande River in 1846 in order to obtain by force the western territories that Mexico refused to sell. History affords few clear examples of ruthless realism and use of power, which brought about the annexation of the northern half of Mexico in 1848. Before World War II, American interventionism was often overt and direct in this way, simply landing troops on the shores of some prospective banana republic and installing a 'friendly' government there. This is exactly what happened in Hawaii, Cuba, Puerto Rico, the Philippines, Haiti, Honduras, Nicaragua and the Dominican Republic, in some cases more than once. Although the "men who directed these 'regime change' operations may not have explained forthrightly why they were acting, but they took responsibility for their acts" (Kinzer 5). Theodore Roosevelt was hardly shy about admitting that he sent troops to Puerto Rico and the Philippines, taking Panama from Columbia or landing in person with the army in Cuba in 1898. Indeed, more sober and pragmatic imperialists feared that he was too much of a 'cowboy' and perhaps unsound in his boasting about a new American Empire. During the Cold War, overthrowing governments was more commonly done in the shadows through CIA covert operations, but once the Soviet Union ceased to exist, the U.S. increasingly reverted to the older methods of direct intervention.
Sebastian Rosato notes that peace may exist among democracies or liberal empires like those of France, Britain and the United States, but their entire record in the Global South is anything but peaceful. As evidence, he offers a long list of colonial wars such as the Opium War of 1839, numerous wars against the indigenous peoples of Australia, New Zealand and the Americas, imperial takeovers of India, Burma, Indochina, the Congo, the Philippines and Indonesia by liberal Western powers, none of which seemed overly concerned with the "rights of non-Europeans" (Rosato 589). In this respect, Cold War and post-Cold War interventions in Haiti, Panama, Sierra Leone, Iraq and many other countries are part of the same continuity in the imperial foreign policies of liberal, democratic states. With Cold War interventions in Chile, Guatemala, Iran and Guatemala the U.S. "did not treat fellow democracies with trust or respect" (Rosato 591). For U.S. foreign policy, the evidence that this is indeed the case is overwhelming, and easily demonstrated by some of the better known examples of interventions and occupations in Hawaii, Central America and the Caribbean long before the Cold War.
Far from promoting democracy abroad, the United States has reacted many times against the threat that democracy poses to American investments and corporate interests. In overthrowing democratic governments and supporting corrupt and pliable elites in Asia, Central America and the Caribbean, "no nation in modern history has done this so often, in so many places so far from its shores" (Kinzer 2). Almost always the real motivation for this is "economic reasons -- specifically to establish, promote and defend the right of Americans to do business around the world without interference" (Kinzer 3). Corporations have been the dominant influence in U.S. politics since the late-19th Century, and foreign leaders who resist them often risk being overthrown. Secretaries of State like John Foster Dulles and Elihu Root have often been agents of Wall Street and large corporate interests within the American foreign policy establishment, which prefers to deal with 'friendly' dictators and corrupt oligarchs rather than nationalistic, populist or radical democratic governments in the developing world.
Even before the war of 1898, Cuban leaders like Jose Marti were rightly concerned that American intervention in their revolution would merely substitute one form of imperialism for another. Far from endorsing the nationalist revolution in Cuba, the U.S. government feared that its proposals for democracy, social welfare and land reform would be a threat to American investments and business interests (Kinzer 37). General Leonard Wood, in charge of the military occupation of the island, was equally blunt in his conception that the meaning of Cuban independence should be openness to foreign investment (Kinzer 42). In addition, the Platt Amendment, passed by Congress along partisan lines, gave the U.S. The power to install and remove governments, supervise its treasury and foreign policy, and to intervene militarily whenever it saw fit. Essentially, it "gave the Cubans permission to rule themselves as long as they allowed the United States to veto any decisions they made" (Kinzer 43). General Wood was not mistaken when he wrote that "there is of course little or no independence left Cuba under the Platt Amendment," but in the end all these actions set the stage for the rise of Fidel Castro in 1959.
From a Native Hawaiian perspective, this story is identical to that of the indigenous people of the Americas since 1492 in that they were also enslaved, exterminated, wiped out by epidemics and marginalized in ghettos and reservations. In 1778 there were over one million Natives, but because of European diseases their numbers fell to 100,000 in the 1840s and just 40,000 by 1890, when they were outnumbered by white and Asian immigrants. Their Native forms of landholding and government were systematically destroyed and replaced by an oligarchy of white plantation owners, who brought in 'coolie' labor from Asia to perform menial agricultural tasks. These planters took control of the government in 1887, denied the vote to Hawaiians and ceded Pearl Harbor to the U.S. "in exchange for duty-free sugar" (Trask 11). No Hawaiians ever had a chance to approve the annexation of the islands by the United States and none had the right to vote for the new territorial government that replaced the monarchy, no more than blacks on Southern plantations or indigenous people on reservations could vote. They simply became a colony of the United States, a subject people, and so they have remained ever since.
Haiti was occupied by the United States Marines occupied it from 1915-34, and in realty was just one of many countries in Central America and the Caribbean that were colonies of the United States in everything but name. For President Woodrow Wilson, Secretary of State William Jennings Bryan and other political and military leaders, who hardly bothered to conceal their racist views toward blacks, the U.S. Marines were "to take up the role of father to what was considered a child nation" (Renda 304). Haiti was in fact the second independent republic in the Americas and also an example of the most successful slave rebellion in history. Thomas Jefferson and other Founders of the U.S. were all too well-aware of this, however, and feared that it would be an example for slave rebellions in the South.…