Can anyone explain the theory of modern American imperialism and its effects in places such as Latin America, Middle East etc?

Theories about American imperialism tall broadly into two categories: (1) those that rest on parallels between American imperialism and empires in the past - most commonly Britain, Rome, and the US's operations in China and the Philippines in the late 19th and early 20th century, and (2) economic theories that argue that there is a modern imperialism shrouded in economic institutions. The first category are more prominent in popular imagination in the West, while the second are more seriously considered in academia.

Probably the most widely debated work on modern imperialism is Negri and Hardt's Empire. Combining Marxist, Weberian, and postmodern theory, Negri and Hardt argue that the US controls the world indirectly and behaves in every way as an empire without officially spreading its flag or borders. Critical to their theory are three ideas: Weberian authority, biopower, and capital accumulation.

The first of these is the idea that authority and power come in multiple forms:

If we wanted to take up again Max Weber’s famous three part formula of the forms of legitimation of power, the qualitative leap that Empire introduces into the definition would consist in the unforeseeable mixture of(1) elements typical of traditional power, (2) an extension of bureaucratic power that is adapted physiologically to the biopolitical context, and (3) a rationality defined by the ‘‘event’’ and by ‘‘charisma’’ that rises up as a power of the singularization of the whole and of the effectiveness of imperial interventions

Hardt and Negri throughout their book strive to prove that the United States possesses all three of these in great quantities throughout the world, not just in its own borders.

The second of these, bipower, is a postmodern concept stating that by controlling everyday behaviors, the powers that be exercise a great amount of control over the population without having to forcefully intervene. Foucault, an earlier postmodern philosopher, used the example of factories in the 19th and 20th centuries - workers were introduced to a rigorous working routine that created thoughtless habits, usually without the need for constant coercion from the government or from their employer.

The third of these concepts, capital accumulation, rests on Marxian theories that capitalists will constantly strive to bring down wages and increase profits. Marxists traditionally believed this was the cause of recessions - as when wages were suppressed, consumption decreased - though this concept of "creative destruction" figures less prominently in Negri and Hardt's work. Instead, the authors use the idea of capital to explain the motives of the alleged ruling class of the United States.

Others works focusing on modern imperialism, like David Harvey's The New Imperialism incorporate these three ideas to some extent. The basic idea is that the United States is the lynchpin of a wider financial system, consolidated by international institutions like the IMF and World Bank, solidified by agreements such as GATT/WTO, making capital freer than ever to move, and, in the eyes of the authors, exploit. Because of the control that the Western world and Japan have over world markets, and the biopower that corporations exercise through marketing campaigns and business structures, it follows that an international financial class has imperial-like control over the global population without having to plant a flag in someone else's country. This quote in particular from Harvey illustrates his point with regards to how this alleged empire has affected the developing world:

But it was a peculiar feature of this world that an increasingly transnational capitalist class of financiers, CEOs, and rentiers, should look to the territorial hegemon to protect their interests and to build the kind of institutional architecture within which they could gather the wealth of the world unto themselves. This class paid very little heed to place-bound or national loyalties or traditions. It could be multi-racial, multi-ethnic, multicultural, and cosmopolitan. If financial exigencies and the quest for profit required plant closures and the diminution of manufacturing capacity in their own backyard, then so be it. US financial interests were perfectly content to undermine US hegemony in production, for example. This system reached its apogee during the Clinton years, when the Rubin-Summers Treasury Department orchestrated international affairs greatly to the advantage of rentier interests on Wall Street, though they often took very high risks in doing so. The culmination was the disciplining of competition from East and South-East Asia in 1997-8 in such a way as to allow the financial centres of Japan and Europe, but above all the United States, to snap up assets for almost nothing and thereby augment their own profit lines at the cost of massive devaluations and the destruction of livelihoods elsewhere. This was, however, only one example of the innumerable debt and financial crises that afflicted many parts of the developing world after 1980 or so.

In Harvey's eyes, the US is the "enforcer" for an international financial system that rests on forcing reforms and openness on the developing world, which then create situations of dependence. Harvey's theory appears to be influenced by Immanuel Wallerstein's ideas on the "Core" and "Periphery" in developmental economics - the idea that capital rich countries with diversified economies, through cycles of investment, competitive advantages, and consumption, trap "peripheral" and less developed countries in resource-producing roles, unless they are able to institute protective tarriffs and other trade control measures which allow them to diversify their economies.

Though not a work analyzing American imperialism, Goto-Shibata's Empire on the Cheap, analyzing British opium exports to China in the 1920s and beyond, draws interesting parallels between the British Empire and modern imperialism. The British Empire is probably the closest parallel in function to the modern day theories of American Empire. Notoriously built "on the cheap", the British Empire at one point ran the entire territory of Anglo-Egyptian Sudan with only several hundred people in the colonial office. They did not interfere in the fine details of local customs, but strove to control and open markets. In China in particular, the British eyed for market control over the Yangtze without seeking to plant their flag on the river.

The 1925 and beyond period is especially interesting, because at that point the British had long abandoned militaristic gunboat diplomacy against Chinese governments. Instead, the British retained their sphere of influence over the Yangtze according to Goto-Shibata with a mix of bribery, investments, and deals encouraging Chinese officials to become more friendly to British investment and business. These closely parallel the developmental deals struck by the IMF with developing countries in Africa and Latin America, where debt relief and development are granted in exchange for political and economic reforms.

Now this is where the debate gets deep - there is considerable disagreement among proponents of the new empire theories as to whether the US is a master or another slave. Certainly the US is the lynchpin in the international financial system, but David Harvey believes that the US is just another slave to capital, and that aggressive moves against states isolated from the international trade system - Iraq being his prime example - that indebt the United States to international capital serve the interest of an international commercial elite. This elite includes everyone from European financiers to the Chinese diaspora in Southeast Asia. These ideas are similar to earlier Marxist thinking: for example, the Great Soviet Encyclopedia in the 1980s wrote that the US had reached the "highest form of capitalism", but fundamentally was just driven around by markets, and was basically the capitalist elite's attack dog.

Despite their Marxian influences, Hardt and Negri believe the US is a master because of the US government's control over the immense capital flows that pass through its markets:

The New Deal produced the highest form of disciplinary government. When we speak of disciplinary government, we are not referring simply to the juridical and political forms that organize it. We are referring primarily to the fact that in a disciplinary society, the entire society, with all its productive and reproductive articulations, is subsumed under the command of capital and the state, and that the society tends, gradually but with unstoppable continuity, to be ruled solely by criteria of capitalist production. A disciplinary society is thus a factory-society.

Because the US government has so many of the regulatory responsibilities in the international financial system - maintaining the international reserve currency, regulating the largest markets, and headquartering numerous international financial institutions - Negri and Hardt believe that the US government is actually in control of the tentacles of international capitalism, not the other way around.

TLDR: People theorize an American empire because of the control the international financial elite have through marketing and enforcing routines over our lifestyles. By opening markets, some allege that the West and East Asia keep the rest of the world in a cycle of resource dependency. There's arguments over whether the US is just the biggest appendage of a system controlled by an international elite, or whether this elite do the bidding of the US due to the US government's control over international finance.