Over the holidays, the House of Cornelius rode down a side trail into the Wars of the Roses.

The Wars of the Roses were a series of civil wars that ripped England apart for 30 years from 1455 to 1485. The complexity of three-decades of instability, turmoil and extreme violence has been over-simplified into a dynastic struggle between the Houses of Lancaster and York, the Red Rose vs. the White. The struggle has been fodder for dramatists from Shakespeare to Philippa Gregory’s “The White Queen,” and George R.R. Martin plundered its rich, bloodshot vein in the fantasy epic that became HBO’s wildly popular “Game of Thrones.”

For all its inherent drama, perhaps the most compelling aspect of the period is its resonance. Historian Dan Jones, who wrote an excellent single-volume history titled “The Wars of the Roses: The Fall of the Plantagenets and the Rise of the Tudors,” prefers the term “resonance” to the often-glib assertion of historical “relevance.”

Seeking on-the-nose, one-to-one correlations between past and present events can easily mislead. But resonance is a different matter — it gets at the continuity of human behavior, the noble and the sordid. It allows us to see ourselves in the people of the past — and to see them in us.

What tore England asunder in a welter of blood was a crisis of legitimacy. King Henry VI — who inherited the Crown as a young child — fell very far indeed from the mighty tree that was his father, Henry V. He was weak, vacillating, and probably seriously mentally ill. He was the legitimate king, but he wasn’t up to the job.

A sincere effort to do right by the realm on the part of his Queen, the remarkable Margaret of Anjou, was contested by the most powerful noble in England, Richard, Duke of York — who considered himself, not without reason, to be a superior candidate for Lord Protector. The question of who would guide England through crisis dissolved into a brutal and intractable conflict.

For decades, no one could establish full, uncontested legitimacy as King of England – and the realm bled.

The American Constitution was designed to make national institutions much more important than the people who hold office. American presidents are not supposed to be monarchs. American citizens are not supposed to rally to the personal banners of overlords.

But over the 232-year history of the presidency, the chief executive has been allowed to grow more and more powerful, to the point at which recent presidents (of both parties) brag about their ability to rule by decree, through what they benignly call “executive orders.”

And when you have a monarchical presidency, a crisis of legitimacy has explosive potential.

For the past two decades, since the excruciatingly close 2000 Bush vs. Gore election, it has become common practice to challenge the legitimacy of the person who sits behind the Resolute Desk. Even when he won by wide margins in 2008 and 2012, some of President Barack Obama’s opponents falsely claimed that his presidency was illegitimate because he wasn’t born in the U.S. In fact, “birtherism” was Donald Trump’s entre into the national political limelight.

In 2016, many Democrats, appalled by Trump’s unexpected victory, loudly cried that he was “not their president” — though he clearly won the election — and, proclaiming “The Resistance,” assiduously sought his removal from office through investigation and impeachment.

Now we are witnessing the unprecedented spectacle of a sitting president undermining the legitimacy of the very institutions he is supposed to represent, claiming against all evidence that he remains the rightful leader of the nation, and sowing what, for millions of Americans, will be an ardent belief that the new president is not legitimate.

A president urging a state official to “find” votes and to “recalculate” an election outcome should alarm any citizen, whatever their political persuasion. Republican Senator Ben Sasse has rightly called President Trump’s post-election behavior “(pointing) a loaded gun at the heart of legitimate self-government.”

Events of recent weeks have expanded the Overton Window of what can be accepted in American politics.

In his history of the Wars of the Roses, Jones notes that, “Richard III’s usurpation of the Crown had broken every rule of political propriety, and with it, opened new and previously unthinkable possibilities.”

The resonance of history whispers that, when the unthinkable becomes possible, the land is in grave danger.