Lentis/Military Industrial Complex
In his farewell address on January 17, 1961, 34th United States President Dwight D. Eisenhower discussed the importance of the military in the future of the United States as the post World War II era transitioned into the Cold War.
"A vital element in keeping the peace is our military establishment. Our arms must be mighty, ready for instant action, so that no potential aggressor may be tempted to risk his own destruction..."
However even with the looming threat of the Soviet Union, Eisenhower remained wary of the growing powers of the military and it's close relationship with the government:
"In the councils of government, we must guard against the acquisition of unwarranted influence, whether sough or unsought, by the military-industrial complex... We must never let the weight of this combination endanger our liberties or democratic processes."
Eisenhower's coining of the term "military-industrial complex" was used to refer to the symbiotic cooperation between the United States government-controlled military and the defense contractors that sell to it. The significance of the warning is compounded by President Eisenhower's status as both a Republican and as one of only five five-star generals. Ordinarily someone in these positions of power was in favor of the military in any sense, especially in the highest seats of power. The military-industrial complex relationship is mutually beneficial: the military desires advanced weaponry, defense companies seek to survive and expand. Due to the enormous amount of money and weapons exchanged between the two, they can be viewed inseparably and as one of the most powerful and influential groups in the world.
Defense Spending & Perpetual War
Before his political career Eisenhower was an accomplished member of the United States military and earned the rank of five-star general during World War II. Following the end of the war, Eisenhower served as Chief of Staff from November 1945 until February 1948 before he began his campaign for he presidency. During this time, Eisenhower developed an appreciation for the scale of the military during World War II when military spending peaked close to 90% of the total federal spending in the mid 1940's (Figure 1), when the United States transitioned into a war economy. As a fraction of total GDP (Figure 2), the size of the defense industry spiked in both World War I (c. 1919) and World War II (c. 1944) with the second war peaking at almost twice that of the first. Additionally, the defense fraction of the economy never truly recovered back to pre-World War II levels, rising from around 2% to 5% baseline. This increase is due in part to the Cold War, when there was political pressure to continue to scale armaments to compete against the growing USSR. Even after the end of the Cold War the percentage of GDP spent on defense never decreased to the previous 5% level. The industrial infrastructure developed during the World War II eras continued to operate and fund smaller conflicts throughout the 20th century such as the Vietnam, Korean, and Persian Gulf wars.
Eisenhower's warning of "guard against the acquisition unwarranted influence" warns that the defense industry may undemocratically pressure the government to continue to involve itself in military conflicts to drive demand for arms sales and defense spending. There exists evidence to support this idea, as since the beginning of World War II the United States has been at conflict for all but five years . The almost perpetual state of war following the Eisenhower presidency was interrupted by only the years 1976-1978, 1997, and 2000. Economically, this is advantageous for defense providers. The wars against communism fought in the 20th century that were frequently criticized for being long, costly, or even "unwinnable" are a boon for the defense industry. A perpetual demand for weaponry creates a steady stream of revenue. Re-purposing a weapons-manufacturing facility is also a costly and time consuming process, which their owners are likely to avoid as long as there is a demand for weapons. The defense industry is entirely for-profit, and is not necessarily concerned with the ethical implications of any conflict. The defense industry pressures the government through lobbying to promote agendas that seek military solutions to foreign conflicts.  The defense industry also pressures the government to support foreign wars so that the United States may sell their weaponry in support.  The justness of frequent military conflicts has been criticized more frequently in the 21st century.  Defense spending is a non-partisan issue, as both GOP and Democratic candidates are sponsored by defense companies despite the GOP's generally more pro-military stance.
Criticisms of military conflicts in the 21st century in the war on terror mirror the criticisms of the war on communism. US presence in Afghanistan has persisted since 2001, and the US had spent over $450 Billion by 2011
International Perceptions and Relations
The international views on the US military-industrial complex are widely polarized. Some groups have common views related by material interests - they rely on the purchase of US military arms to fuel internal struggles. However, certain nations resent the Western-dominated reach that this complex may afford nations such as the US. Resentment to overarching reach is a common value held by many nations that are not ready for "Western infiltration" into their home nations.
Positive Foreign Markets for the Exchange of US Arms
The United States exports about 33% of all arms on the global market. Saudi Arabia is the top buyer of US arms, purchasing $90 billion worth between 2010 and 2015. The development of large military-industrial nations that buy US arms are attributable to the United State's own investment in offshore-procurement (OFP) during the Cold War.  South Korea, for example, largely expanded it's industrial power during the Cold War era when the United States was greatly expanding long-range purchasing, including OFPs from South Korea. South Korea today is the 4th largest buyer of US arms. Exchange of arms provides benefits for both the United States and the foreign buyer: mutual values are reinforced and economic flow is sustained in both directions. For example, Israeli purchasing of arms reinforce the United State's interest to spread democratic values in the Middle East.  Dennis Ross, Senior Director for the Central Region during the Obama administration, states that "[he] cannot recall a time during [his] public life when [the] two countries have had a closer defense relationship. The U.S. and Israel are cooperating closely in areas such as missile defense technology, the Joint Strike Fighter, and in training exercises such as Juniper Stallion — cooperation and support that ensures that Israel will continue to maintain its qualitative military edge."  In this sense, a similar trend we have observed for intrinsic defense-related ventures are carried into foreign policy relations. A self-sustaining cycle is reinforced initially through mutual values, followed by an ever-growing economic power that holds the two nations together. This type of cycle can be seen in many nations, like that of South Korea and Israel described above, that share similar Western principles of democracy and freedom.
Negative Markets and External Apprehensions
The exchange of US arms to select nations has invoked negative viewpoints of both American military-intervention and capitalist ventures. After the events of September 11, 2001, military spending increased 50% in the following decade.  Those nations with which the US does not seek to conduct business view these sales with contempt. Iran's President Hassan Rouhani, after an attack by an unknown foreign assailant on an Iranian military parade, stated that "All of those small mercenary countries that we see in this region are backed by America. It is Americans who instigate them and provide them with necessary means to commit these crimes", referring to the arms that Americans had sold to countries surrounding Iran. The industrial push for selling of arms has the potential to initiate a "long chain of responses that the United States generally cannot control". 
A significant portion of defense spending goes to fund jobs in technologically advancing fields of research, such as computing, automation, and materials science. The US is projected to have 2.1 million people employed by the defense industry by 2026. Additionally, the funding to the defense industry has directly resulted in rapid technological advancement between World War II and the present. For example, the Internet and GPS are two massive innovations which have strong influence on modern society. The US continues to be the sole source of funds for GPS maintenance and research, despite presence and availability in many other countries. Low power LEDs are a development from the US defense industry, which enables low-heat light generation. It is theorized that the high levels of funding for aircraft research during the 20th century advanced the technology significantly enough for common civilian use by the late 20th century.
The population of the US has long been known to (on some level) subscribe to the ideology of American Exceptionalism.  Innovation, independence, and self-sufficiency are fundamental American values, and these social values are intertwined with the rate of advancement the US has maintained since World War II. This image is also held by non-US citizens. When interviewed by the Associated Press, individuals from several countries explained their own picture of the US. Many of these individuals voiced the US's drive to be ahead in technology as a significant portion of their perception. This perception makes it more difficult to justify lowering defense spending, because the topic implicates individuals' personal perceptions of the US. This follows with the Thomas theorem, which states that the perception of a situation is just as important as the situation itself.
Michael Beckley notes that the US is bound by alliance to protect up to 25% of the world population. This estimate covers all countries the US has a standing treaty or alliance with, as well as soft pledges for aid (such as with Israel). This estimate does not include some groups for which the US has deployed military force, such as citizens of several Middle-Eastern countries. The number of nations that rely on US military protection further complicates the issue of military defense spending.