In short, the United States became more aggressive in attaining foreign resources and access to trade. This was a result of the expansive nature of empires, and the fact that America, as characterized by Boot, was gradually becoming a "great power."
Largely, the Great Powers of the modern world have come into being as emerging economic and political trends have allowed. The ever shifting tides of the world's social foundations have tended to produce successive powers that rise and fall over the course of history. Generally, what make these powers great are their military capabilities, but of course, these are commanded by the economic base supporting them. A powerful economy can allow for enormous military expenditures, and generate vast influence across the planet.
Naturally, the economic crux of a great power can vary in form. Most obviously, the great power of the ancient world -- Rome -- relied upon the slave trade for its riches and influence; but today, in the wake of the industrial revolution, manufacturing tends to demand which nations become great powers and which languish in mediocrity. Basically, wealth and an efficient economy generate the type of nations that hold the potential to become great powers. The key is to balance a nation's wealth with the military expenditures that become inevitable once a country achieves great status.
Greatness means that a country's influence can be felt from across continents, oceans, and time. A great power holds many obligations and has many interests throughout the world. Doubtlessly, power has always been relative. Nations and dynasties have reached their greatness as vacuums of power and influence made themselves evident. Often, serendipitous events have placed specific nations in positions where they had the unique opportunity to make their authority felt on a grand scale, or in ways that were never felt before. The United States came to world dominance -- what Boot calls a superpower -- following the Second World War. Yet, it had been recognized well before that period that the U.S. possessed the capability to achieve such levels of greatness. With the enormous agricultural base inherited by the Louisiana Purchase and subsequent escapades, the United States, like Russia, found its self in a unique social situation; this was a situation that was extremely conducive, if not demanding of growth.
Boot indicates that America's almost natural expansion as a great power occurred in a global setting that was becoming increasingly dangerous for imperialistic nations. This was for two main reasons. First, nations that chose to engage in colonial campaigns began to find, as the Americans did in the Philippines, embroiled in continual and costly insurgent conflicts: "More than three years before, the burgeoning power of the United States had dismembered the remains of the Spanish empire and put the Philippines under the Stars and Stripes. The Filipinos stubbornly resisted their new colonial masters, and though successive U.S. generals proclaimed victory at hand, American soldiers kept dying in ambushes, telegraph lines kept getting cut, and army convoys kept being attacked," (Boot 99). Although the conflict in the Philippines has become one of America's forgotten wars, according to Boot, it offers a great deal of insight into the way in which such conflicts have been addressed by the military. Essentially, America's forgotten wars have all been initially handled as conventional wars; accordingly, the conventional strengths and weaknesses of the enemies were assessed and troops were deployed in a limited capacity. However, the unstable nature of these conflicts eventually demanded more serious involvement.
The second key problem facing the United States as it began to enter the fray as a colonial power was that the colonial nations of the world were beginning to butt heads, and what many believed was an unavoidable war between them was about to drag the United States into the Great War. Imperialism -- which began to truly expand in the 1870s -- guaranteed that as these nations grew more influential, the lands they sought to control would become evermore scarce. Essentially, as the number of valuable colonies dwindled around the world, it grew increasingly apparent that the only way to finalize the global balance of power would be through direct conflict. Since no nations were willing to sacrifice their global might in the aim of international harmony, many were of the belief that war could not be avoided. In short, the cause of the World War I was colonialism; as it would happen, it would remain one of the key causes of World War II.
In the United States, the political stance of Wilson and that of most Americans was that of isolationism. However, once the war began to significantly interfere with American commerce and shipping, it became increasingly difficult for Wilson to remain neutral. Additionally, there was ongoing political pressure from both London and Berlin for the United States to enter the war on either side. Yet, as the war went on, it became German submarine attacks that gradually began to convince many Americans that action needed to be taken against the Germans. After the Lusitanian was sunk, Germany agreed to cease attacks upon merchant vessels; but this ended in 1917 when three American merchant ships were sunk by German submarines. As a result of these attacks, congress declared war upon Germany on April 6, of that year. Ultimately, it was the closer political and economic ties that America held with Britain and France, as well as the type of warfare the Germans were waging that that resulted in the United States declaring war on Germany and its allies.
World War I was the first significant step in transforming the United States -- and Russia as well -- into a global political, economic and militaristic power. Much of this was simply the result of the boosting of manufacturing and production that the war forced the government to subsidize. Of course, technological advancements and innovation came along with this, but the key force was the external political pressure resulting in the near full utilization of the United States' already vast stock of resources. The conflict itself resulted in the expansion of U.S. forces abroad and the correlated extension of American political and economic interests, which could then be facilitated by the already mobilized militaristic force -- such as in the Philippines. This brought the United States on its way toward becoming one of the preeminent global powers; however, since the vast majority of American imperial interests were in the Americas, a relapse into a form of isolationism followed World War I, though it was far less pronounced than prior to U.S. involvement.
Overall, Boot's handling of the United State's small wars during this period of expansion in Central and South America is very forgiving of the goals and methods by which America made its imperialistic power felt abroad. He writes, "Critics of American intervention later charged that the U.S. had deliberately installed dictatorships in Hispaniola and elsewhere in the Caribbean. Very nearly the opposite is true.... Unfortunately, those who came to power were not always of the highest moral caliber. The only thing more unsavory than U.S. intervention, it turned out, was U.S. nonintervention," (Boot 181). Essentially, Boot argues -- although this argument is mostly implicit throughout the book -- that the United States, because of its substantial power and it's basic tenets of freedom and democracy is often morally required to enter into the fray in these small wars. In general, it is contended, American involvement is beneficial, obviously, to America -- when it is successful -- but also to the rest of the world, because it promotes stability, economic freedom, and the ideals of Western democracy.
Following the First World War, the United States continued to exert it's control in Central America and the Philippines, although it did so more economically than militaristically: "It would be an exaggeration to say that America turned isolationist, but there is not question that before the Great War, America's military commitments had been expanding, whereas afterward they contracted.... America resorted once again to dollar diplomacy, with private companies extending loans and investment to Europe, Latin America, the Middle East, Asia, even to the Soviet Union," (Boot 231). However, with the U.S. incursion into Nicaragua, a handful of valuable lessons were learned about guerrilla warfare: "Of course, as the U.S. learned a few decades later, body counts can hardly be the measure of success in a guerrilla war. On one level, the Nicaragua adventure was frustrating; after all, the marines never managed to catch Sandino of stamp-out his movement. Nevertheless they did keep Sandino on the run," (Boot 252). Again, it was a limited engagement that gradually became much larger than American officials had been prepared for. It was possible to prevent the followers of Sandino from using many overt or conventional means of waging war, but it remained…