This is Part III of a three-part commentary series.

So far, U.S President Trump has not offered an answer for the nation to rally behind and to reassure his critics.

In its absence, experts have sought reassurance in the president's fragmentary and sporadic pronouncements to support their own vision. Neo-isolationists have cheered his efforts to close American borders. Others have warmed to the notion that he has suggested our allies assume more responsibility for their own defense. Even proponents of old-fashioned primacy have sought luster by interpreting the president's defense buildup as a return to the unilateralist days of American military prowess through intervention.

Read Part I: What's the purpose of President Trump's Navy?

Read Part II: The service's view

Our own research suggests that the truth is that none of these grand visions may apply. The Navy, and indeed the other military services, face a growing demand for their services. They are now being asked to perform an increasing number of functions that are not associated with fighting wars.

The military even has a term for it: MOOTW (military operations other than war). And the Navy's MOOTW ranges from conventional war fighting against other countries' navies to policing the globe against pirates, drug flows and the smuggling of nuclear materials, humanitarian assistance, and even fighting Ebola in Africa. These activities consume much of the Navy's time. And their increasing demands require increased resources. Military budgets therefore often reflect the requirements entailed in providing these services as much as the need to conform to any one image.

Of course, congressional democrats may yet scuttle plans for an enlarged Navy. Alternatively, the president may move beyond discussing discrete missions to a more coherent grand strategy — perhaps tutored by his new senior military appointments — that justifies acquisition decisions.

The types of ships (and aircraft, unmanned systems and equipment) purchased in the coming years will make sense only if they are employed in an operationally coherent manner. Only then will the American public be able to judge if the trade-offs made to fund such an enterprise were worth it.

Simon Reich is a professor of global affairs and political science at Rutgers University. Peter Dombrowski is a professor of strategy at the Naval War College.