The Regulatory Structure

Two federal agencies share jurisdiction over airline competition. The Department of Justice (DOJ) Antitrust Division has held lead authority over domestic airline mergers since 1989, when Congress transferred that jurisdiction from the Department of Transportation (DOT) via Public Law 100-223, Section 203, 49 U.S.C. § 41709.

The DOT retained separate authority under 49 U.S.C. §§ 41308–41309 to approve international alliance agreements and grant antitrust immunity (ATI) to carriers coordinating fares, schedules, and capacity on international routes. The DOT maintains a public database of alliance and codeshare agreements.

Both agencies enforce the same underlying statutes. Section 7 of the Clayton Act (15 U.S.C. § 18) prohibits mergers and acquisitions that may substantially lessen competition. Sections 1 and 2 of the Sherman Act (15 U.S.C. §§ 1–2) prohibit anticompetitive agreements and monopolization, respectively. The DOJ and Federal Trade Commission (FTC) jointly issued the 2023 Merger Guidelines on December 18, 2023, which remain in effect under the current administration per memoranda from FTC Chair Andrew Ferguson and DOJ Acting AAG Omeed Assefi dated February 18, 2025.

How DOJ Analyzes Airline Mergers

The DOJ defines relevant airline markets as city-pair routes—the origin and destination a passenger needs to travel—rather than as the industry as a whole. In certain cases, the relevant market narrows further to nonstop service between specific airports. The DOJ evaluates whether a proposed transaction would raise the Herfindahl-Hirschman Index (HHI) above thresholds that trigger a presumption of illegality under the 2023 Merger Guidelines.

Entry conditions receive particular attention. Four airports in the United States operate under FAA slot controls: DCA, JFK, and LGA operate under Level 3 mandatory slot allocation; EWR, ORD, LAX, and SFO operate under Level 2 schedule facilitation. At these airports, slots or scheduling coordination constrain new carrier entry and become the binding constraint on competition.

The DOJ treats slots as a "license to compete." In its March 2015 challenge to United's proposed acquisition of 24 additional slot pairs at Newark (EWR)—where United already controlled 73% of allocated slots—the DOJ emphasized that antitrust remedies in slot-controlled airports must preserve competitive access by transferring slots to potential competitors.

The DOJ also analyzes whether merging carriers operate "alternate hubs"—hubs in the same general region that provide competing connecting itineraries for smaller city pairs without nonstop service. This concern has informed DOJ review since at least the 1990s.

Domestic Market Concentration

Four carriers—American Airlines, Delta Air Lines, Southwest Airlines, and United Airlines—collectively held approximately 74% of U.S. domestic seat capacity as of September 2025, based on OAG scheduling data. American Airlines held an estimated 21% share, Delta 19%, Southwest 18%, and United 16%, per Spherical Insights (September 2025). Alaska Airlines, following its $1.9 billion acquisition of Hawaiian Airlines completed in September 2024, holds approximately 5%.

This four-carrier structure dates to the 2013 American Airlines–US Airways merger, which reduced the number of hub-and-spoke national systems from four to three legacy networks plus Southwest's point-to-point model.

Recent Enforcement Actions

Four DOJ and DOT actions since 2021 illustrate the current analytical framework and its outcomes:

American Airlines–JetBlue Northeast Alliance (2021–2025)

In September 2021, the DOJ and six state attorneys general filed suit under Sherman Act § 1 to enjoin the Northeast Alliance (NEA), a joint venture under which American Airlines and JetBlue coordinated schedules, shared revenue, and pooled operations on routes in and out of Boston and New York.

U.S. District Judge Leo Sorokin (D. Mass.) ruled in May 2023 that the NEA's anticompetitive effects—reduced capacity and higher fares on overlapping routes—outweighed any procompetitive benefits. On appeal, the First Circuit affirmed in November 2024. On June 30, 2025, the U.S. Supreme Court denied certiorari in American Airlines Group v. United States, No. 24-938.

The DOJ under the current administration supported the government's position in the case, arguing the First Circuit applied "uncontroversial antitrust principles."

JetBlue–Spirit Airlines Merger (2023–2024)

In March 2023, the DOJ sued to block JetBlue's proposed $3.8 billion acquisition of Spirit Airlines under Clayton Act § 7. The DOJ argued that eliminating Spirit's ultra-low-cost carrier (ULCC) model—which the court termed the "Spirit Effect"—would remove downward pressure on fares across the domestic market.

U.S. District Judge William Young (D. Mass.) permanently enjoined the merger on January 16, 2024, finding that cost-conscious travelers who rely on Spirit's pricing structure would be harmed.

Spirit Airlines subsequently filed for Chapter 11 bankruptcy in November 2024, emerged in March 2025 after an 87-day balance-sheet restructuring, and filed a second Chapter 11 petition on August 29, 2025. Spirit reduced domestic capacity by 22% year-over-year as of September 2025.

Alaska Airlines–Hawaiian Airlines Merger (2023–2024)

Alaska Airlines' $1.9 billion acquisition of Hawaiian Airlines cleared the DOJ's HSR Act review period in August 2024 without a challenge. The DOT approved the transaction in September 2024, subject to conditions including preservation of both carriers' mileage programs, maintenance of inter-island routes and Honolulu-to-mainland service, family seating policies, and military family discounts.

The different outcome compared to JetBlue–Spirit reflects the DOJ's market-by-market analytical approach: Alaska and Hawaiian operated limited overlapping nonstop routes, and neither carrier functioned as a ULCC exerting discipline on the broader fare structure.

Delta Air Lines–Aeroméxico Antitrust Immunity (2016–2025)

The DOT granted antitrust immunity to the Delta–Aeroméxico joint venture in 2016, subject to conditions including slot divestitures at Mexico City International Airport (MEX) and JFK, removal of exclusivity clauses, and a five-year term. When the carriers applied for renewal, the DOT issued a preliminary decision declining to extend immunity. On August 11, 2025, the DOJ filed comments supporting that decision, citing "restrictive and potentially discriminatory practices by the Government of Mexico" that limited carrier entry and expansion at MEX.

The DOT issued a final order terminating approval in September 2025 under 49 U.S.C. §§ 41308–41309. The JV accounted for approximately 21.8% of two-way U.S.–Mexico seats as of August 2025, per CAPA data.

Remedies in Airline Antitrust Cases

When the DOJ permits a merger to proceed subject to conditions, structural remedies—primarily slot divestitures—are the preferred tool. The logic: at capacity-constrained airports, slots are the binding constraint on new entry, and transferring slots to low-cost carriers creates the conditions for post-merger competition.

The largest airline divestiture to date accompanied the 2013 American Airlines–US Airways merger. The settlement required divestiture of 52 slot pairs at DCA, 17 slot pairs at LGA, and gates at five additional airports (Boston Logan, Chicago O'Hare, Dallas Love Field, Los Angeles, and Miami), all to DOJ-approved low-cost carrier purchasers. The 52 DCA slot pairs represented all air carrier slots held by American prior to the merger.

In earlier cases, the DOJ required slot divestitures in the United–Continental merger at EWR (2010) to address United's concentration at that airport. Sixteen peak-hour slots were divested to Southwest Airlines as a condition of the merger.

In international alliance proceedings, the DOT has required route carve-outs (excluding specific competitive overlap routes from immunity) and slot divestitures. The Delta–Aeroméxico 2016 ATI grant required divestiture of slots at MEX and JFK to out-of-market low-cost carriers.

The DOJ/DOT Joint Request for Information (October 2024)

On October 24, 2024, the DOJ and DOT jointly published a Request for Information (RFI) on the state of competition in air travel, covering airline consolidation, airport access, aircraft manufacturing, ticket sales and pricing, and rewards programs. Comments were due December 23, 2024.

Airport access was identified as a standalone topic, with the agencies seeking information on how access conditions affect "airlines and their ability to enter and fairly compete in different areas of the country and the world." The RFI was initiated under the Biden administration; the extent to which the current administration will act on the responses has not been announced as of March 2026.

Implications for Airport Operators

DOJ and DOT antitrust enforcement creates direct effects on airport operations, finances, and planning. Areas where the antitrust framework intersects with airport management include:

Slot and gate divestitures alter the carrier mix. When the DOJ requires slot divestitures to low-cost carriers as a merger condition, the acquiring airport's tenant base changes. At DCA following the American–US Airways settlement, 17 destinations lost year-round daily nonstop service from American as a result of the 52 slot pair divestitures. Airports with cost-per-enplanement (CPE) rate structures tied to residual or compensatory methodology may wish to evaluate how carrier-mix changes affect airline cost allocation.

Antitrust immunity decisions affect hub traffic volumes. Research examining transatlantic route data found that the granting of antitrust immunity to alliance partners was associated with a 4.1% to 11.5% decrease in non-alliance carrier passenger volumes at immunized hub airports, as excluded carriers reduced service frequency and shifted to smaller aircraft. The DOT's September 2025 withdrawal of Delta–Aeroméxico ATI may alter traffic patterns on U.S.–Mexico routes that flow through airports serving as connecting hubs for either carrier.

Merger outcomes shape revenue risk for airports with concentrated carrier exposure. Spirit Airlines' trajectory from blocked merger (January 2024) to first Chapter 11 filing (November 2024) to second Chapter 11 filing (August 2025) illustrates how antitrust enforcement outcomes can affect an airline's financial viability and, by extension, the airports it serves.

FAA slot waivers interact with antitrust enforcement. The FAA extended its limited waiver of slot usage requirements at DCA, JFK, and LGA through October 24, 2026, in response to air traffic control staffing constraints at the N90 TRACON facility. These waivers allow carriers to return up to 10% of slots without forfeiture—an interaction between operational constraints and the competitive access that antitrust remedies are designed to protect. The FAA notice states that DOT will monitor for "unfair, deceptive, or anticompetitive practices" associated with the waivers.

Current Administration Posture

The current DOJ Antitrust Division has taken a position of continuity on airline antitrust enforcement in certain respects. The 2023 Merger Guidelines remain in effect. The DOJ supported the government's trial victory in the American–JetBlue NEA case through Supreme Court proceedings in June 2025. Deputy AAG Dina Kallay stated in November 2025 that antitrust immunity "should be narrowly construed and carefully considered, particularly in light of the public benefit standard Congress provided to the DOT."

At the same time, the first Trump administration (2017–2021) adopted a "laissez-faire approach to mergers and joint ventures," and the DOT during that period approved the American–JetBlue NEA in January 2021, days before the transition of power. Both FTC Chair Ferguson and Acting AAG Assefi have acknowledged that the 2023 Merger Guidelines may be subject to future revision through an "iterative and transparent" process.

The direction of future enforcement depends on multiple variables, including whether the DOJ/DOT act on the October 2024 RFI responses, how courts apply the 2023 Merger Guidelines in subsequent airline cases, and whether DOT exercises its independent authority under 49 U.S.C. § 41712 to address unfair methods of competition—an authority it has used sparingly in the airline context.