Big Tech’s day of reckoning has finally arrived.

Both of the major antitrust enforcement agencies of the executive branch of the federal government are stepping into the fray, with the Department of Justice reportedly preparing for an investigation of Google parent Alphabet, Inc. and the Federal Trade Commission (FTC) doing the same for social media giant Facebook.

The decision by President Trump and Attorney General William Barr to clear their regulators for action is a watershed moment, both in the history of antitrust law and in the evolution of Silicon Valley’s relationship with American society. After years in which the Obama administration delayed and thwarted any major action against the Big Tech companies, the decision has apparently even shamed some House Democrats into playing along.

This change of tack is long overdue; Google and Facebook are undoubtedly massive, manipulative monopolies.

Google controls roughly 90 percent of the online search market. Three quarters of mobile and 70 percent of desktop web browsing takes place on Google software, and its YouTube subsidiary has completely dominated the user-generated streaming video market virtually unchallenged since its inception.

Facebook’s eponymous service alone makes up more than two-thirds of the entire social media market, and it owns the rapidly growing Instagram photo-sharing service, too.

The two companies together make up more than half of the total American online advertising market, and they’re rapidly branching out into new markets, such as self-driving cars and virtual reality technology.

It’s impossible to ignore the anti-competitive threat that this unchecked market dominance poses, especially given Big Tech’s increasing willingness to exclude and punish firms that do not conform to its vision of internet speech and conduct. And yet, until now, Google and Facebook have avoided any significant antitrust scrutiny.

The last time antitrust regulators seriously examined Google, in 2012, FTC investigators concluded that major anti-competitive conduct was at play that “has resulted — and will result — in real harm to consumers and to innovation.”

President Obama’s political appointees at the FTC, however, nixed any meaningful sanctions, and Google had backchannel discussions with the White House, where dozens of former Google employees and associates worked. The issue was laid to rest.

That reckless inaction cannot be separated from Silicon Valley’s disturbingly close relationship with the Democratic Party and the broader political left. The vast majority of Google and Facebook’s political cash, not to mention the contributions made by its employees, flows to the Party that is its staunchest defender on Capitol Hill.

Then-Alphabet Chairman Eric Schmidt, for instance, was intimately involved in Hillary Clinton’s run for President, even wearing a “staff” badge to her thwarted victory party. Even now, key Democrats such as House Judiciary Committee Chairman Jerry Nadler get their biggest corporate donations from Google, Facebook, and the other tech giants.

In return for political protection, Big Tech has delivered increasingly blatant and muscular enforcement of the political left’s priorities. In the latest outrage, Facebook appears to have revealed a private user’s identity to liberal journalists who wanted to punish him for making a meme out of House Speaker Nancy Pelosi.

But the cause for concern over Big Tech’s power goes far beyond politics.

The scale and power of these firms is staggering. Google and Facebook have a combined market capitalization of about $1.2 trillion, roughly equal to the gross domestic product of Mexico. If you include Apple and Amazon — the other members of Silicon Valley’s “Big Four” — we’re talking about a market cap equivalent to the GDP of the United Kingdom, the world’s fifth-largest economy.

The tech giants manage more aspects of your life than any private entities in history and collect enough data about the average citizen to rival even the most intrusive totalitarian secret police forces. It’s virtually impossible to live as a private citizen — let alone run an internet-related business — without being subjected to the terms and conditions of Google and Facebook on a daily basis.

That enormous economic power goes hand in hand with political and social might that Silicon Valley shows a disturbing willingness to flex in defense of totalitarian foreign regimes. It would not be an exaggeration to say that the tech giants are on the verge of wielding significant global political power in their own right.

Addressing that kind of concentrated economic power is a central purpose of antitrust law. Yet, with few exceptions, Big Tech has operated with a complete lack of serious oversight.

The benefits our society accrued from their freewheeling disruption — the “move fast and break things” ethos at the core of tech innovation — were treated as an excuse for looking the other way. Afraid to kill the golden goose, we’ve been too slow to reexamine that bargain.

It’s long past time for society to start evaluating Google, Facebook, and the other tech monopolies with a more skeptical eye. It’s a credit to President Trump that his administration is finally doing so.