Which US allies have paid their bills, as President Donald Trump would see things? Which, having given the United States little support in return for its security guarantee, now risk losing it?

The short answer, derived from our numerical methodology, is that only nine countries in the US’s main European and Indo-Pacific alliance networks are genuine net contributors to their partnerships with Washington. Australia, Britain and the Netherlands rank highest. Poland, Norway and France are also pulling their weight.

Sixteen countries in those alliances, though not quite free-riders, can fairly be called cheap-riders, according to our assessment, which measures allies’ commitments of blood and treasure. Another 12 may be classified as blatant cheap-riders, notably including Japan, which has the largest economy among the US’s friends.

Our assessment does not focus on Washington’s Latin American and Caribbean allies, but, if it did, they’d all be classed as cheap-riders or blatant cheap-riders.

With Trump taking the unprecedented step of linking protection with payment, our analysis aims to clarify allies’ risks of US abandonment. For the NATO and Indo-Pacific allies, this is no mere academic exercise. European NATO members face an aggressive Russia that has threatened to expand its war against Ukraine. And US allies in the Indo-Pacific confront an increasingly assertive and powerful Beijing, alongside growing nuclear and missile threats from Pyongyang.

Contrary to expectations, we found that proximity to these threats did not necessarily correlate with higher contribution to the US alliance, especially in Europe.

Within alliances that are asymmetric, as any with the US must be, weaker partners cannot fully compensate the stronger partner for protection. They’re not rich enough. But they can contribute (or, in Trump’s parlance, ‘pay’) through such actions as providing international diplomatic support, forward bases or niche military capabilities.

Trump generally attaches greater weight to more readily quantifiable measures, such as defence spending as a percentage of GDP. So we follow him, answering the bottom-line question ‘Who’s paid?’ by asking five component questions with readily quantifiable insights. We aggregate the results into an overall payment score.

First, has the ally met its defence spending targets over the lifetime of the alliance? Washington expects allies to spend at least 2 percent of GDP on defence (though Trump has floated higher standards). By doing so, allies develop properly funded independent military capabilities, reducing the US’s burden of guaranteeing their security. Higher spending also makes them more useful potential partners in US-led coalitions operating outside the alliance areas. Consistently meeting the 2 percent target, amid constant pressures on the public purse, also demonstrates a domestic political resolve that enhances the alliance’s deterrent potential. So we assess lifetime spending by comparing each ally’s total defence expenditure and GDP during its time in alliance with the US. Net contributors meet the 2 percent threshold, whereas net cheap-riders fall short.

Second, has the ally met its defence spending targets over the past decade? Military capabilities, accrued over time, atrophy without sufficient ongoing funding. Washington, for example, built a world-class navy in the American Civil War—which, after years of underinvestment, amounted to just ‘an alphabet of floating washtubs’. Correspondingly, recent defence spending provides insight into which allies have maintained the military capability and preparedness that Washington values. And, again, it shows political resolve. We assess recent spending by considering allies’ defence expenditures and GDPs since 2015 (when combat operations in the last US-led ground-war ended and when Trump’s full engagement in politics began). Net contributors meet the 2 percent threshold, whereas those falling short have either been persistent cheap-riders or, having formerly paid their dues, have now decided to take it easy.

Third, how much US weaponry has the ally purchased? Allied acquisitions of US military equipment, such as aircraft, give Washington several benefits: revenue from and longer production runs of existing systems (for example, F-16s); more work from their maintenance programs; savings from cooperative development of new systems (such as the F-35); and improved US and allied fighting strength thanks to the ease of operating common equipment. We assess weapons purchases by considering allies’ relative shares of US arms transfers and global GDP during their alliance tenure. Scores under 1 indicate comparatively limited purchases, whereas those exceeding 1 denote outsized purchases, and those above 2 show purchases that greatly favour US suppliers.

Fourth, has the ally supported US-led combat coalitions? Allied participation in military operations benefits Washington by providing international legitimation for the action and reducing the burden on the US. Alliances, however, are not wellsprings of guaranteed support: as self-interested actors, allies can decline to render aid or even defect to opposing blocs. Correspondingly, joining US-led coalitions builds good faith with Washington (and implicitly serves as down payment on reciprocal assistance). We assess participation by considering five ground-war coalitions (those for the wars in Korea, Vietnam, Persian Gulf, Afghanistan and Iraq) and five primarily air-war coalitions (in the Iraqi No-Fly Zones and campaigns in Bosnia, Kosovo and Libya and against ISIS). We allocate points according to the burden undertaken: for ground-wars, 8 points for providing frontline combat forces, 4 for supporting units, and 2 for financial assistance. For air wars (which involve less cost and risk), point values are halved. We count allies as consistently supportive if their points exceed 17 points and as reliable combat partners if they exceed 30.

Fifth, has the ally paid a blood price? Allied personnel losses, incurred while furthering Washington’s security interests, represent a shared sacrifice, one that demonstrates the highest form of loyalty (a value cherished by Trump) and implicitly serve as further down payment on reciprocal assistance. Since US-led air wars have featured minimal casualties, we assess losses by counting the number of US-led ground wars after World War II in which allies have suffered service deaths.

We generate overall payment scores by aggregating allies’ performances across all five measures. Each measure receives a 20 percent weighting, and we grant maximum points for:

—Meeting the 2 percent defence expenditure target during the period of alliance;

—Meeting it in the past 10 years;

—Greatly favouring the US in weapons purchases;

—Providing frontline combat forces for each US-led combat coalition; and

—Incurring personnel losses in each US-led coalition ground war.

Partial points are awarded relative to these maximums. Scores below 50 indicate blatant cheap-riding. Those exceeding 70 denote genuine net contributors—for example, 40 for meeting both spending targets, 20 for joining and suffering losses in more US-led coalitions than not, and 10 for outsized weapons purchases.

So, who’s paid?

The US alliance network contains few genuine net contributors, with only nine of 38 NATO and Indo-Pacific allies exceeding 70 points. Moreover, three net contributors deserve qualification: Greece and Turkey generally prioritise each other as a threat rather than NATO’s common adversary, Russia, and South Korea owes the US for its ongoing protection along with its defence during the Korean War.

The Indo-Pacific allies contribute relatively more than their NATO counterparts, averaging higher overall and component scores (apart from participation in operations, among which were three NATO-centric air-war coalitions). Compared with NATO, the Indo-Pacific alliance network also includes a greater percentage of genuine net contributors (28 percent versus 22 percent) and a much lower percentage of blatant cheap-riders (14 percent versus 35 percent).

Notable cheap-riders include Germany and Japan, because they have large economies and therefore great potential military might.

It’s also remarkable that cheap-riding is common in the countries of NATO’s Eastern European expansion. Apart from Poland, Romania and the Baltics, all are blatant cheap-riders, even though their membership has brought added burdens and risks to the alliance, including the US.

Australia is well insulated against Trump’s potential revisions to US alliance policy, which largely (and, in light of our findings, rightly) concentrate on redressing NATO’s relative underpayment. Canberra is immune to similar charges: no other ally has given Washington comparatively more blood and treasure than Australia, and the Albanese government has already begun reversing recent dips in defence spending, pledging to spend 2.3 percent of GDP by 2034. Moreover, Australia’s ‘indispensable’ strategic partnerships with other US allies remain relatively safe: Britain ranks second in terms of its alliance contributions (which bodes well for AUKUS solvency), and Japan, though a definite laggard, has been steadily boosting what Trump would see as its payments. It’s greatly lifting defence spending, increasing host-nation financial support and reinterpreting its constitution to permit collective military action.

How, or whether, Canberra’s unrivalled contributions will affect its bargaining position with Washington remains to be seen and needs supplementing with qualitive analyses (as given here for the first Trump presidency).