As a global hegemon, the United States is a leader in the market of international security. Its military is the best funded, trained and equipped in the world, and should therefore be abundantly ready to manage a range of actual and potential threats to national and international security. It is nonetheless sometimes underprepared for the challenges the country faces once it decides to commit forces to a conflict.

Why is this the case?

One significant reason is because defense choices made today—or in the past—constrain future options, require the acceptance of risk and compel adversary adaptation, which in turn reduce the value of investments initially made to maintain or improve military readiness.

Defense budgets are finite and choices have to be made. The United States cannot prepare equally for every possible threat to its national security. Because it cannot prepare for every possible threat or contingency, the United States must accept some level of risk in respect to the types of adversaries it expects to face and the operations it expects to conduct. In addition, its adversaries are increasingly capable and adaptive, which gives them an unofficial vote in how conflicts involving U.S. forces unfold. The more adaptive the adversary, the more likely it will confound readiness investments made previously to confront it.

Defense budgets and planning decisions regarding weapons-system purchases, troop levels, and the construction of doctrinal concepts and requisite training regimens all require significant investments of time and money. Once made, these decisions tend to be difficult to reverse or even modify.

For example, the development of what resulted in the Bradley M2 Infantry Fighting Vehicle took decades to complete, and the B-52 Stratofortress that has been in service for more than a half a century is expected to remain in service for a few more decades.

With respect to troop levels, increasing or decreasing the size of the force is a process that takes years to actualize. Reducing force size—which happened at the end of the Cold War and is happening again now—is difficult because downsizing usually entails a loss of capability and experience that is not easily replaced. Skilled and capable forces cannot be created out of whole cloth.

Military doctrine and associated training are also difficult to adjust once established. For instance, although the conflicts in Afghanistan and Iraq quickly devolved from force-on-force operations to counterinsurgent struggles, doctrine to guide these kinds of operations and associated training programs did not emerge until 2006.

Once significant and durable defense decisions are made, they naturally reduce the force's capacity to adapt to changes in the international security environment when they occur. Because the United States cannot afford to prioritize and defend against every possible contingency, it must therefore accept risk—strategic, operational and even tactical—with each decision it makes. For instance, if the United States prioritizes and funds a capacity to deter and defeat insurgents and other violent non-state actors—as it has for the past 15 years—it must then accept risk in respect to its readiness to deter and defeat other types of threats to security such as near-peer threats..

The investment decisions the United States prioritizes and the risks it chooses to accept set the conditions for how it can and will operate in future conflicts. Unfortunately, the choices made also set the conditions for how its adversaries will choose to adapt and respond.

Nation-states, such as China and Russia—as they have in the past with U.S. nuclear weapons development, naval deployments, or advancements in its space program— are keeping close watch on how the United States arranges and deploys its forces in Europe, the Middle East and the Pacific. Non-state actors, such as the Islamic State and al-Qaida and its affiliates, are also taking stock of how the United States deploys forces, conducts operations and collects intelligence in its fight against terror organizations and insurgencies in the greater Middle East and North Africa.

Because both sets of actors—states and non-state organizations—have greater access to manifold means for and methods of conducting warfare than they did in the past, they are able to adapt to the defense decisions the United States makes in ways that confound calculations of military readiness.

This convergence of threat capabilities and modus operandi is making potential and actual adversaries much more adaptive. This, in turn, is making it that much more difficult for the United States to resource a military that historically has been expected to prepare for one type of threat or the other, with state-based adversaries—as the more consequential of the two—justifiably receiving the bulk of military planners' attention. Although recent conflicts in Iraq, Afghanistan and elsewhere have tempered this expectation, it is nonetheless still a prominent feature of military planning and judgments of military readiness.

Regardless of the adversary, the United States is being required more often than not to adapt to maintain its readiness levels—threats to national and international security are no longer static.

The more significant the adaptation required (either tactical or strategic), the more time and money it will take and the more reluctant or unable defense planners will be to make these adjustments. If, as is happening more frequently, an adversary can change its operational methods and postures quickly and substantially within the scope of a conflict, it will make it very difficult for the United States to maintain desired readiness levels for any significant period.

Improving future military readiness will require significant adjustments to not only how readiness is conceived of and measured, but also to how units are organized and trained. Lessons learned through experiences in Afghanistan and Iraq regarding how, and the pace at which, military units and institutions adapt to changing operational environments and adversaries should not be forgotten, as the United States reconfigures its defense posture after 15 years of conflict. Ultimately, reducing strategic, operational and tactical risk, and improving military readiness will require conceiving of defense choices in a way that does not constrain future options and the capacity to adapt, but instead expands them.

Chad C. Serena and Colin P. Clarke are political scientists at the nonprofit, nonpartisan RAND Corporation.