The antitrust scrutiny of tech giants that began during the Trump era will only intensify this fall as Big Tech critics Lina Khan, Tim Wu and Jonathan Kanter take the lead on competition policy and enforcement in the Biden administration.

Why it matters: Facebook, Google, Amazon and Apple face threats from federal regulators, Congress, state attorneys general and European Union authorities.

The big picture: That's four companies each being challenged from four directions: No wonder the antitrust arena can feel like three-dimensional chess.

As the fall season looms, here's what the game board looks like:

Facebook

The Federal Trade Commission, now led by Khan, renewed its legal effort challenging Facebook's acquisitions of Instagram and WhatsApp in August. The FTC accuses Facebook of buying rivals or using anticompetitive tactics to stymie them in order to squelch competition.

• What to watch: Facebook has until Oct. 4 to respond.

The European Commission launched an antitrust investigation of Facebook Marketplace in June over concerns that Facebook's collection of data from advertisers gives it an unfair advantage.

• What to watch: The United Kingdom announced a similar investigation in June that also focuses on Facebook's online dating service.

In Congress, the House Judiciary Committee narrowly approved a slate of tech antitrust bills, including one that would force more interoperability and another that would bar big companies from snapping up rivals through acquisitions.

• What to watch: Bipartisan companion legislation in the Senate would give these bills some momentum. Sen. Tom Cotton (R-Ark.) said in July he intends to introduce a bill that would curb mergers among big tech companies.

Amazon

The FTC has been investigating Amazon's business practices since the Trump administration and is also digging into the e-commerce giant's plan to buy Hollywood studio MGM.

• What to watch: Amazon wants Khan to recuse herself from FTC's Amazon cases, given her previous advocacy of action against the company.

The European Commission accused Amazon last November of violating antitrust rules by harnessing data it collects from third-party sellers to shape the products it offers that compete with those merchants.

• What to watch: The commission also opened a separate investigation into how Amazon selects which products get the coveted "Buy Box" label. But a Financial Times story in March suggested that case has been an uphill climb.

In Congress, Amazon faces the potential for drastic changes to its business model through the House antitrust bills that would bar it from both operating its online marketplaces and selling goods on them.

• What to watch: Amazon is warning sellers that they could bear the brunt of the cost if such legislation is enacted — and hoping those sellers will call their representatives.

Google

The Justice Department and several state attorneys general filed multiple antitrust lawsuits against Google last year, with the DOJ accusing Google of an illegal monopoly in online search and search advertising.

• What to watch: The judge in DOJ's case indicated it likely won't go to trial until 2023. President Joe Biden nominated Jonathan Kanter, an antitrust attorney who has battled Google on behalf of its tech foes, to lead the antitrust division of the DOJ, though he has not yet been confirmed by the Senate.

In Congress, Google faces multiple legislative threats, from the House antitrust bills as well as legislation in both the House and the Senate that would curb its power over its Google Play Store.

• What to watch: State attorneys general also sued Google over how it operates its app store.

The European Commission opened its own investigation in June into Google's power in the online advertising ecosystem.

• What to watch: Previous European antitrust investigations into Google have led to billions of dollars in fines.

Apple

In Congress, Apple is facing proposed laws in both House and Senate that would limit its control over how it runs its App Store.

• What to watch: Apple recently offered some concessions on its App Store policies to settle a class-action lawsuit — but not enough to satisfy those who back these bills.

The European Commission, acting on a complaint by Spotify, accused Apple in April of violating antitrust laws by requiring rival music streamers to use its in-app payment system and follow other rules.

• What to watch: The commission opened a separate investigation in June to more broadly review Apple's rules for app developers.

The Justice Department isreportedly also investigating Apple for anticompetitive practices, although that probe has led to no charges so far.