Polymarket's rules count American air or missile strikes on Yemeni land and would not count U.S. support for Saudi operations, naval gunfire or strikes by allies. Traders price about a 20% chance of such a strike by Oct. 31.
As Washington weighs whether to join Saudi retaliation against Yemen's Houthi forces, a Polymarket market on the question is asking something narrower than whether the U.S. enters the fight.
Polymarket's contracts titled "US x Yemen ceasefire continues through [date]?" resolve No if "the United States takes a qualifying military action against Yemen by the specified date," measured at 11:59 p.m. Yemen time.
What counts
A qualifying action is a U.S.-initiated air strike, using bombs, air-to-surface missiles or air-launched drones, or a surface-to-surface strike, using one-way attack drones or cruise or ballistic missiles, that hits Yemen's land territory or internal waters.
The rules exclude intercepted munitions and debris, surface-to-air missiles, small arms, ground incursions, cyber operations, naval gunfire, artillery and rockets, and short-range loitering munitions. Strikes by contractors, proxies or non-U.S. coalition members do not count. In a joint operation, only strikes the U.S. carries out itself qualify. Threats that are never carried out do not count.
That means American intelligence, refueling or other support for Saudi strikes would not resolve the contract. The market can stay open for up to three days after a first credible report if accounts conflict.
The prices
The contract for a ceasefire holding through Oct. 31 traded at 79.5 cents on Monday afternoon, down from 80.5 cents early in the morning. That implies about a 20.5% chance of a qualifying U.S. strike by the end of the month. The Oct. 15 contract rose to 94.5 cents from 93.5, implying about 5.5% by Thursday. The Oct. 12 contract, which settles Monday, was near 98 cents.
The gap between the two dates is informative. Of the roughly 20.5 cents of strike risk priced through Oct. 31, about 15 cents falls after Thursday, a much longer window. Per day, traders price slightly more risk in the next few days than later in the month.
The Oct. 31 market is thin, with less than $80,000 traded.
The backdrop
Riyadh's main airport has been struck four times in a week, and about 60% of Monday's flights were expected to be canceled, according to aviation-data provider Cirium. Saudi Arabia's aviation authority says attacks on the kingdom's airports from Oct. 6 to Oct. 11 killed 20 people and injured 356. Saudi Energy Minister Prince Abdulaziz bin Salman said the kingdom's response must be "equally serious."
President Trump said on Saturday that the U.S. "may" join Saudi strikes and would decide quickly. No decision had been announced by Monday afternoon.
Market implications
One reading is that the price is low because direct U.S. action is a large step. Supporting Saudi operations would let Washington help without the political cost of its own strikes, and that path would not move the contract.
A second reading is that one in five in less than three weeks is a meaningful risk, and that traders see it as immediate rather than distant. The market prices slightly more risk per day before Thursday than after it, consistent with a decision that could come at any moment from a president who says he moves quickly.
What to watch
Any White House announcement on Saudi support, the form that support takes, further attacks on Saudi airports or energy sites, and whether the Oct. 15 contract holds above 94 cents will move the prices.