Historic moments are memorable, but not always for the happiness they bring. America is truly living through a sad time, one that finds a nation that has rarely been more politically polarized.

The congressional inquiry into the exchanges between President Donald Trump and the newly elected president of Ukraine will be divisive, surely, as critics have charged. It will overtake legislative priorities that, instead, merit lawmakers’ full attention.

But it will be as necessary as it is ugly, a hideous chapter of American history, but one on which the nation can’t afford to turn the page.

The aim at this stage should be merely to gain as much clarity as possible about what citizens have a right to expect of their chief executive — and what should be found intolerable.

And what should citizens demand? That the president fulfill his constitutional duties, using government resources to protect national security and the public interest — not his own political ends.

In order to discover whether Trump has indeed fallen short of his mandate, an inquiry potentially leading to articles of impeachment is entirely justified, whether or not lawmakers in the end find that the allegations have merit.

Trump has been accused of freezing some critical military aid Congress set aside for Ukraine; that country needs the funds to help in its fight against pro-Russian separatists.

The president’s intent, House leadership and others assert, was to use the $400 million as a cudgel, driving Ukraine officials to dig up dirt on a potential electoral rival: former Vice President Joe Biden. The president hasn’t yet produced evidence of wrongdoing by Biden or his son, a board member for a Ukrainian company.

Even more alarming was this allegation: The Trump administration has worked to suppress a whistle-blower report of the troubling situation. The informant, believed to be a member of the intelligence community, aired concern that Trump had pressed his Ukrainian counterpart for oppo research on the Bidens for personal political gain.

The potential breach was considered urgent. Ultimately the report was released to Congress, as it should have been from the start.

With good reason, House Speaker Nancy Pelosi had initially resisted the calls for impeachment that swirled in the midst of the investigation into obstruction and other charges against the president, led by Robert Mueller, the former FBI director and special counsel. She believed there wasn’t enough support, both within her Democratic caucus and among the public, for such an action.

She was right then. But in the wake of the whistle-blower story, and seeing more of her politically moderate members get off the fence, she decided to endorse the ongoing investigations as being part of an impeachment probe.

Pelosi’s change of heart still poses considerable political risk, as the public remains largely unmoved by a congressional push toward impeachment.

But the facts that have come out so far, incomplete as they are, do suggest that Congress could not continue to avert its gaze. At the very least, the House committees already convened for the inquiry must work to answer the question: What really happened between Trump and Volodymyr Zelensky?

Zelensky is a novice politician, an entertainer whose popularity swept him into office in May. His principal concern is the defense of his country, and the U.S. foreign policy remains in support of Ukrainian security.

That is the reason Congress appropriated the military aid, which Zelensky was desperate to receive. He was supposed to receive it over the summer, in fact, but Trump decided to put it on ice, according to some reports.

For his part, the president insists he did not put pressure on Zelensky, who during a United Nations meeting last week said as much in a public statement. But it would be hard to take that at face value, given that Ukraine’s subordinate rank to the U.S. put Zelensky in an untenable position.

What is needed is a clear-headed examination of the facts, one that’s focused on this particular case. If the assertions are borne out, they would underscore an abuse of presidential power.

This is anything but the first time a sitting president has been accused of such things. During President Barack Obama’s two terms, Republicans decried what they considered abuses of his “imperial presidency,” and multiple investigations ensued. Similarly, Presidents George W. Bush and Bill Clinton had the spotlight turned on them as well.

All four members of Hawaii’s congressional delegation are now unanimous, in favor of the inquiry. As divisive as this probe might be, it’s warranted.

The founders of this nation intended for the executive branch powers to be considerable, but not limitless, so the winning argument is the one for bringing the facts into the open. Oversight is, after all, the role that the Constitution gives to Congress, and it must be defended.