Prior to the mid 20th century the pattern of US military spending (and operations) was to expand during wartime and contract during peace time. This was certainly true throughout the majority of the 19th century, and you can see this through the progression of The US Civil War, for example, which started with a mad scramble to ramp up to sufficient levels of men and materiel to prosecute the war. To some degree this pattern was an intentional creation, with the founders very leery of large standing armies during peacetime.

However, things began to change around the turn of the 20th century or so with the creation of the "Great White Fleet", though even then typical peace-time military spending in the US was only around 1% of GDP. In the inter-war years this changed pretty dramatically to be around 2% of GDP, though that didn't represent a huge change in peace-time war-fighting capacity, it mostly represented the increasingly expensive nature of industrialized warfare. By the 1920s and '30s the US had trucks and planes and aircraft carriers and massive battleships, etc, etc, etc. All of that costs money. And things like battleships and aircraft carriers typically have very long timeframes for procurement, taking years to build and put into service.

Even prior to WWII the US had transitioned to a stance of maintaining a level of war fighting capability comparable to the other "great powers", and this was reified in international treaties such as the Washington Naval Treaty which put the US and UK naval forces at a 1:1 level of parity in terms of total tonnage while all other nations participating in the treaty (such as Japan, France, and Italy) were allowed lower levels of total tonnage.

Of course, the examples of both WWI and WWII illustrated the extent to which American industry could ramp up and support truly unprecedented levels of production of war-goods and armaments, with WWII-era America in particular being a major example of a near complete transformation of the economy to put it on a "war footing" and to optimizing for production that supports an all out war effort. Immediately after WWII ended defense spending fell dramatically, but not to previous levels. This was a time when the US was actively occupying a significant fraction of the world (much of Western Europe as well as Japan and Korea), it had also acquired a new level of military equipment which were yet more expensive to maintain, develop, and build than from the interwar years (with jet aircraft, rocket propelled guided missiles, nuclear weapons, and so on).

It's also easy to forget that through much of the early "Cold War" the US was still engaged in "hot" wars. With the start of the Korean War the US was caught a bit flat footed in not being able to respond quickly with sufficient war fighting capability, that provided a lesson that justified higher levels of peace-time defense spending throughout the rest of the Cold War. During the Korean War US military spending hit around 15% of GDP and then fell back to around 10% through the rest of the 1950s as the Cold War had started in earnest. A substantial amount of this spending was in maintaining a standing army capable of "force projection" across the globe fairly rapidly, the origin of the modern US military with multiple fleet carrier battlegroups and divisions of land forces kept at the ready. But a good chunk of it was also dedicated to rapidly cycling through R&D on different generations of technology. A lot of systems in the early Cold War only saw service history of a few years as they were developed and brought into service before being rapidly obsoleted (sometimes intentionally) by later generations of technology. This was also the time when the US massively ramped up nuclear weapons production and nuclear weapons delivery systems (including ICBMs, IRBMs, SLBMs, and strategic bombers). This was a time of massive technological change as well as uncertainty about geopolitical stability, through the 1950s the Soviets developed not only nuclear weapons but also thermonuclear bombs and ICBMs, and the US became incredibly paranoid about the possibilities of a devastating first-strike assault. Throughout the 1960s US military spending was around 10% of GDP, fully 10x the 19th century norm and about 5x the pre-WWII norm. But this supported a feverish level of activity. Constant active patrols of nuclear armed strategic bombers (operation Chrome Dome et al) as well as the build-out of vast ICBM and nuclear missile submarine forces. It also includes spending on the Vietnam War as US involvement ramped up.

After the ending of US involvement in Vietnam defense spending falls to somewhere between 5 and 10% as the Cold War settled into its late stage level of quasi-stability. Reagan pushed up military spending a little with various programs (such as the "600 ship navy" plan) but that was comparatively minor next to proper war-level spending. At the end of the Cold War there was some level of draw down of deployed military forces and maintained equipment which led to a brief "Peace Dividend" amounting to hundreds of billions of dollars a year that helped contribute to a momentary blip in US federal budget surpluses in the late '90s (substantially helped along by the first "dot com" boom). However, that was a short-lived trend as the 9/11 attacks transitioned into the "war on terror" era which is relevant but outside the scope of this sub-reddit since it falls within the last 20 years.