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THURSDAY, AUGUST 27, 2026   |   SPECIAL REPORT & ANALYSIS

SPECIAL REPORT  |  U.S./CANADA TRADE

Greer Warns Canada Against Escalation as Trade War Hardens

Ottawa’s Sept. 8 retaliation raises risk of a broader U.S. tariff response

Analysis  ·  August 27, 2026

U.S. Trade Representative Jamieson Greer is explicitly warning Canada that its planned retaliation could trigger another U.S. response, adding a new escalation risk to a trade dispute that only days ago appeared close to a negotiated settlement. In an interview Wednesday with CBC’s The National, reported by The Hill, Greer said the Trump administration would not simply accept additional Canadian retaliation and summarized Washington’s position as essentially: “don’t retaliate.” His warning comes as Canada prepares to impose tariffs of 15%, 25% and 50% on C$27.6 billion, roughly US$20 billion, of U.S. goods beginning Sept. 8.

The increasingly important point is that Washington and Ottawa now disagree not only about the underlying trade practices but about whether Canada has a legitimate right to retaliate at all. The Trump administration portrays its original tariffs as corrective measures against Canadian discrimination and therefore views Canadian countermeasures as another offense warranting a U.S. response. Canada sees the same sequence in reverse: Washington imposed new duties, so Ottawa believes proportionate retaliation is necessary to defend Canadian economic interests.

That difference creates a potentially self-reinforcing tariff cycle.

Table 1. How the U.S.-Canada tariff dispute escalated. Sources: White House Section 338 proclamations; Canada Department of Finance; CBC; Reuters.

The latest U.S. duties took effect Aug. 22, not Sunday, after the administration briefly postponed their original Aug. 19 starting date while negotiations continued. The 50% additional tariffs cover selected Canadian products under Section 338 of the Tariff Act of 1930, including goods affected by U.S. complaints involving Canadian dairy, alcoholic beverages and vehicle policies. Canada says its Sept. 8 countermeasures will match Washington “dollar for dollar, rate for rate.”

Table 2. What the U.S. Section 338 action covers. USMCA preference does not exempt these goods; products already subject to Section 232 duties and certain civil-aircraft parts are excluded. Import values are 2024. Source: White House proclamations; White & Case analysis.

Greer’s threat is more important than it initially sounds

There is an important legal constraint on what Trump can do next through Section 338: the statute caps additional duties at 50%, meaning the administration is already using the maximum tariff rate available under that particular provision on the targeted products.

That means Greer’s threat probably should not be interpreted simply as 50% becoming 75% or 100% under Section 338 on the same goods.

Instead, Washington has several other escalation paths.

It could expand the number of Canadian products covered by Section 338, bringing a larger share of bilateral trade under the 50% surcharge. Section 338 also gives the president authority, under certain circumstances, to go beyond tariffs and exclude Canadian products from the U.S. market if the discrimination is maintained or increased. Alternatively, the administration could rely on other statutes — including Section 232 national-security tariffs or other trade authorities — against additional Canadian industries.

Table 3. Washington’s escalation options and their ceilings. Sources: Tariff Act of 1930, Sec. 338; Morrison Foerster and Holland & Knight analyses; U.S. Census Bureau trade data.

And the largest potential escalation remains autos. Trump has already threatened substantially higher tariffs on Canadian vehicles and parts beginning in 2027. Because North American auto production is deeply integrated, that would represent a much larger economic shock than the approximately US$20 billion of Canadian trade covered by the latest Section 338 action.

Thus, Greer is effectively warning Ottawa that Sept. 8 may not mark the end of the retaliation cycle — it could become the trigger for the next U.S. tariff package.

Agriculture is increasingly exposed

The dispute also matters considerably more to U.S. agriculture than the overall US$20 billion headline suggests.

Canada was the second-largest market for U.S. agricultural exports in 2025, purchasing $28.2 billion, or about 16.7% of total U.S. agricultural exports. Major U.S. sales included ethanol, pet food, beef, live cattle, pork, fruits, vegetables and other processed foods. The two countries’ agricultural industries are unusually integrated after nearly four decades of progressively freer trade under the Canada-U.S. Free Trade Agreement, NAFTA and USMCA.

Canada’s new retaliation directly reaches parts of that relationship. Ottawa’s list includes U.S. dairy products and agricultural machinery, with some dairy tariff lines facing 50% duties and machinery facing rates ranging from 15% to 25%.

Table 4. Canada’s Sept. 8 counter-tariffs on U.S. farm, food and equipment lines. Export values are the most recent full year. Source: Canada Department of Finance counter-tariff list. 

There is evidence, however, that Ottawa is trying to avoid hurting itself unnecessarily. Canada on Thursday removed fish and seafood products from the Sept. 8 retaliation list, suggesting the government is willing to modify its countermeasures where Canadian processors, consumers or importers would bear too much of the cost.

That could become important for agriculture generally. Tariff retaliation works politically only as long as the pain imposed on the other country exceeds the domestic cost. Given the integration of U.S.-Canadian food, livestock, fertilizer and machinery supply chains, both governments will have strong incentives to create exemptions once individual industries begin documenting disruptions.

Potash remains one of the biggest risks — and restraints

One particularly sensitive agricultural input remains Canadian potash. Canada is by far the dominant foreign supplier to U.S. farmers, giving Ottawa potentially significant leverage.

Yet Saskatchewan Premier Scott Moe has cautioned against using potash or oil exports as weapons in the trade fight because restricting them would also damage Canadian producers and encourage U.S. buyers to seek alternative suppliers. For now, those products remain outside the latest escalation.

That restraint is important. A move involving potash would quickly transform the dispute from a targeted tariff fight into a much more direct U.S. farm-cost issue, particularly ahead of the 2027 planting season.

The deeper casualty may be USMCA

The most consequential development may not be any individual tariff. It is the weakening of the assumption that USMCA protects qualifying North American trade from tariffs.

The new Section 338 duties reach some products that previously benefited from preferential treatment under USMCA. That sharply increases uncertainty for companies that built North American supply chains around the expectation that meeting USMCA rules of origin would provide reliable tariff-free access.

The dispute therefore risks turning the 2026 USMCA review into something considerably larger than a technical review of the agreement. Washington is signaling that compliance with USMCA alone may not shield Canada from separate U.S. trade remedies, while Ottawa increasingly questions the value of concessions if Washington can subsequently impose duties using other statutes.

That is potentially more damaging to investment than the current tariff bill. Manufacturers can absorb a temporary tariff or rearrange sourcing around a narrow product list. It is much harder to make multibillion-dollar investment decisions when companies cannot be certain what North American market-access rules will apply several years from now.

There are still small signs of an off-ramp

Despite the rhetoric, there are two modest indications that neither side has completely abandoned a negotiated solution.

Greer clarified this week that Canadian French-language and cultural policies were not themselves a deal-breaker, an explanation Canadian Minister Dominic LeBlanc publicly welcomed Thursday. But Reuters also reports that there is currently no open negotiating channel between the governments, underscoring how far relations have deteriorated since last week’s talks collapsed.

Canada’s seafood exemption provides another indication of flexibility.

Those developments suggest the more probable near-term path may be selective exemptions and quiet adjustments rather than an immediate comprehensive settlement.

Bottom line

Greer’s warning significantly raises the stakes surrounding Sept. 8. Canada has deliberately scheduled its tariffs more than two weeks after the U.S. duties took effect, leaving a window in which the measures could theoretically be modified, postponed or used as negotiating leverage. But there are currently no formal talks underway, and neither government has much political incentive to appear to retreat first.

The central risk is therefore an escalation ladder:

U.S. Section 338 tariffs → Canadian Sept. 8 retaliation → additional U.S. action → broader Canadian retaliation.

For agriculture, the immediate exposure is concentrated in dairy, agricultural equipment and selected food products, while the much bigger risks would come if the dispute eventually spreads to potash, energy, autos or a broader range of agricultural trade.

And because Section 338 is already at its statutory 50% tariff ceiling, Greer’s comments point toward something potentially more consequential than simply raising rates: Washington could broaden the trade war itself.

That makes the period between now and Sept. 8 the next critical window in the U.S./Canada dispute.

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AG POLICY & MARKETS DAILY   |   SPECIAL REPORT  |  U.S.-CANADA TRADE — THURSDAY, AUGUST 27, 2026