(The opinions expressed here are those of the author, a columnist for Reuters.)

By Peter Apps

LONDON, March 21 (Reuters) - As Donald Trump and Kamala Harris faced off in the 2024 U.S. presidential race, Estonian and Taiwanese troops were training together at Fort Sill in Oklahoma to use U.S.-made HIMARS long-range rocket systems their governments had purchased.

Those military sales and the associated training were part of a worldwide effort by the then-Biden administration to tie together the U.S. and its allies, something Biden's Defense Secretary Lloyd Austin termed a global "convergence" when it came to planning and technology to confront a growing autocratic axis of China, Russia, North Korea and Iran.

This week, those authorities and military commanders will be looking at those weapons systems - and a host of others - with much greater nervousness after Trump's administration shut down intelligence sharing with Ukraine for several days in a way that rendered HIMARS rockets there temporarily ineffective.

Allied states are still digesting the scale of those implications. But in Europe, both the European Union and its constituent member states are pledging a massive rearmament effort explicitly aimed not just at deterring Russia but ending decades of dependence on the United States.

Europe's apparently unprecedented rearmament initiative including a 150-billion-euro ($162.5 billion) EU defence package, a move with profound implications for the bloc, its member nations and European arms manufacturers.

"We need to see not only Russia as a threat, but also ... more global geopolitical developments, and where Americans will put their strategic attention," European Defence Commissioner Andrius Kubilius told reporters this week.

That represents a massive change.

Ever since Russian President Vladimir Putin's full-scale invasion of Ukraine in 2022, European nations have been working through NATO with the U.S. military to devise sophisticated defence plans in case Moscow subsequently acts to attack an alliance member in eastern or northern Europe.

Part of those plans, defence sources say, involve demonstrating an ability to hit back at Russia hard - but these are heavily reliant on U.S.-made rockets and F-35 jet fighters.

They are also dependent on U.S. intelligence-sharing, air-to-air refuelling and electronic warfare capabilities, all of which could be withdrawn if the U.S. so wished.

But it is the spillover risk of weapons systems already purchased by U.S. allies - including Japan, Australia and South Korea - ceasing to work that has those allies really flustered.

"The Europeans are discovering that if the U.S. doesn't want to back their forces, they are quite vulnerable," said Sten Rynning, a visiting lecturer at NATO's defence college and professor at the Danish Institute for Advanced Study.

"Almost all allies have bought a lot off the shelf from the United States to tie themselves into the (NATO) alliance and show solidarity," he continued.

"(But) if the U.S. wants to turn off, to disable European equipment, then it can. It can refuse to send the software upgrades ... It can refuse to send the spare parts. It can refuse to send the intelligence on which they depend."

That fundamentally gives America's closest allies a relatively simple choice - they can either look to build their own truly "sovereign" capability without U.S. dependence, or they can continue to embrace U.S. technology partnerships while working to mitigate the risk of finding themselves abandoned.

"Donald Trump has scared the ***t out of U.S. allies," a former U.S. official with dealings in Europe and the Middle East told me on condition of anonymity. "But some people are excited by the opportunities."

In the short and medium term, plenty of major U.S. allies - including Britain, Poland, Australia, Japan, South Korea and beyond - are likely to want to keep both options open.

The European Union and some of its most powerful states, however, have increasingly talked in recent weeks as if they wish to step entirely away from U.S. dependence.

In a move that would have previously seemed unthinkable, new Canadian Prime Minister Mark Carney - his country embroiled in a worsening feud with Washington as Trump repeatedly rumbles about making it the 51st U.S. state - looks to be seriously considering throwing in Canada's lot with the EU, perhaps even cancelling its next tranche of F-35 Joint Strike Fighters.

TECHNOLOGY PLUS RHETORIC BRINGS WORRY

Perhaps unsurprisingly, the stock prices of non-U.S. defence companies have soared this year even as other shares have slumped as Trump has pursued global trade wars against allies and adversaries alike.

South Korea's Hanwha Aerospace, which sells artillery to Europe, has seen its stock more than double since January. Germany's Rheinmetall has seen its value double since last year, and expects orders to grow as much as another third by the end of 2025.

Even building one's own weapons systems does not necessarily guarantee the ability to operate them seamlessly.

The British Storm Shadow and French Scalp missiles, effectively the same Anglo-French system under different brands, have been rendered unusable in Ukraine due to Russian jamming without additional U.S. tech to help them reach their targets.

There are clearly ways around that.

This week, just before Trump spoke to Putin late on Tuesday, Ukrainian President Volodymyr Zelenskiy announced that Kyiv had developed a long-range strike drone with a range of three thousand kilometers (1,860 miles).

French President Emmanuel Macron has also been publicly reminding both Europe and the Kremlin that the French nuclear deterrent is "French from end to end", entirely independent of any other power - and that he might use it to defend other European nations.

Ever since Charles de Gaulle in the 1960s, French leaders and military thinkers have periodically warned that the U.S. might one day abandon Europe, particularly in a crisis. De Gaulle questioned whether a future U.S. president would truly risk the nuclear annihilation of U.S. cities like New York to defend European ones like Paris from occupation or destruction.

Last weekend, Macron told several European newspapers he intended to launch a charm offensive to "go and convince European states that have become accustomed to buying American" to buy European instead.

That included pushing those currently looking at buying U.S. Patriot rockets from Lockheed Martin ( LMT ) - soon to be built in Germany - to choose French-Italian SAMP/T instead.

It also included encouraging those considering further purchases of the F-35 - which is built by a multinational conglomerate but largely U.S.-led - to instead purchase the less sophisticated French Rafale fighter jet.

During the temporary shutdown of U.S. intelligence support for Ukraine, which also appeared to affect the operation of its F-16s, French and Ukrainian engineers worked frantically to get French-built Mirage jets into the air to bring down Russian missiles. This was a not-so-subtle advertisement for French and more broadly European-built technology.

Pan-European missile manufacturer MBDA is also working on a longer-range, thousand-kilometer missile for France, Germany, Italy and Poland, although the time scale remains unclear.

FRENETIC NEWSFLOW

European worries over dependence on specific systems, particularly when it comes to satellite communications, have been simmering since 2022, when Elon Musk's Starlink was reported to have refused Ukrainian access to its systems to support strikes on the Russian-held Crimea region.

In the aftermath of that, Taiwan - increasingly jittery about a potential Chinese invasion - reportedly struck deals with other satellite operators to retain access in a crisis.

More broadly, though, to a greater extent than other governments, Taiwan has purchased U.S. military equipment and signalled it is prepared to buy even more under the Trump administration, which took office in January.

But what has happened with Ukraine, with Trump appearing to tilt towards Russia's side, has intensified worries in Taiwan.

In the short term, much depends on how the Trump administration handles the next negotiations with Russia and Ukraine. The more Washington is seen to be swinging back behind Ukraine, the happier other allies will be, while they will worry more if Trump is seen siding more with Putin.

America's allies are now nervously beholding a Trump administration they believe is looking to divide the world back into great power "spheres of influence".

They are concerned Washington is now more open to Moscow and Beijing dominating Europe and Asia, respectively, provided the U.S. is unchallenged in its own hemisphere, where Trump has made clear his designs on the Panama Canal, Danish-administered Greenland, and Canada.

While the shutdown of U.S. intelligence and military aid for Ukraine lasted only a few days, its effect on long-term U.S. relationships with allies may prove much more durable.

Throughout the Biden administration - indeed, through much of recent history going back to World War Two - the U.S. has prodded allies to buy U.S. systems to deepen relationships and ensure that they can fight together.

Allies most closely aligned with Washington have tended to be tightlipped in recent weeks: Japan, Britain, South Korea and Australia being particular examples of nations that remain very keen to retain good relations with the Trump administration.

Still, in South Korea, Japan and Australia, the last three weeks have seen a quiet intensification of debate as to whether they might require their own nuclear weapons in the future. In Britain, there is talk of "diversifying" the UK nuclear deterrent away from just the U.S.-built Trident missile.

However, Britain this week re-committed itself to the Trident programme as it steps up construction of new generation submarines to carry the U.S.-owned ballistic missiles.

Where that might go if the current pace of change driven by Trump's upending of longtime alliances continues is difficult to predict.

"Everything is up in the air, or whatever phrase you want to use," a European defence source told me earlier in the month on condition of anonymity.

A Baltic contact was rather blunter. "Everyone is scared and it is because of Trump," she said. ($1 = 0.9228 euros) (By Peter Apps; editing by Mark Heinrich)