The UK's decision to allow Chinese tech firm Huawei to play a limited role in its 5G networks is one that took years to make, and one which Britain says was based on a technical and security analysis that offered "the most detailed assessment in the world." It's clear that the country thinks this is a move based on evidence, but the impact of it will be closely watched by its allies on both sides of the pond.

For the U.S., this was a serious blow. Washington spent months lobbying the UK and urging it not to work with Huawei, saying it would leave the country vulnerable to spying by the Chinese state. British intelligence agencies found no evidence of that, and said any future risks could be mitigated.

But there's plenty of anger in the U.S. on both sides of the political divide: Democrats and Republicans alike have tweeted their disapproval.

Republican senator Tom Cotton tweeted: "This decision is deeply disappointing for American supporters of the Special Relationship. I fear London has freed itself from Brussels only to cede sovereignty to Beijing." Likewise, Senator Lindsey Graham - a key figure in President Trump's party - said "This decision has the potential to jeopardize U.S.-UK intelligence sharing agreements and could greatly complicate a U.S.-UK free trade agreement."

Read more EU 5G advice: No ban on Huawei in latest blow to U.S.

Those are the two key areas the UK will be worried about in terms of any blowback.

Firstly, will this decision affect the Trump administration's intelligence partnership with London? Their current security relationship is thought to be the most solid in the world. The two countries are also members of the 'Five Eyes' security bloc - along with Australia, New Zealand and Canada. Canberra and Wellington have already followed the U.S. in banning Huawei, so this move does risk a division.

The other main lever the U.S. has at the moment is around a free trade deal. Britain is set to leave the European Union on Friday, and its biggest priority aside from a trade agreement with the EU is one with Washington. 

Anger over the Huawei move could see the White House take a tougher stance on the negotiations. As we've seen with the U.S.-China trade deal, President Trump often sees foreign policy in binary terms - you're either with us or against us.

As for the EU, its response to this decision by the UK is likely to be very different. Like London, Brussels has been resisting pressure from the U.S. in recent years to completely turn its back on the Chinese company.

Many member countries, including France and Germany, already use Huawei infrastructure and ripping that out would be costly; Britain considered a similar argument in making its own decision. As things stand, Huawei's 5G technology is some of the most advanced in the world, and crucially, some of the cheapest too. Those will be compelling reasons working in the company's favor across Europe.

Read more Huawei threatens UK sovereignty, Pompeo warns ahead of 5G decision

The public response from the Europeans so far has been muted, but Angela Merkel has previously said Germany's decision on using Huawei would be based on its own interests. 

France and Germany are waiting to announce their own decisions on Huawei and the UK's stance could influence what they do. In particular, UK Prime Minister Boris Johnson is selling this as a compromise - allowing Huawei to continue providing equipment in the UK, while labelling it a "high-risk vendor" that can only have 35 percent of market share in the "non-core, periphery" networks.

But, like much of politics in this globalized world, there will be other considerations for the EU, namely, growing antagonism with the U.S. over trade issues. It was on European soil in the Swiss ski resort of Davos this month that President Trump warned Europe to ramp up efforts to agree a trade deal or risk facing "very high tariffs."

Against this complicated backdrop, U.S. Secretary of State Mike Pompeo begins a trip to London on Wednesday before flying to Ukraine on Thursday. Britain's decision has set the scene for a showdown between London and its closest ally. 

This could be an issue that tests the so-called special relationship. What happens next will be closely watched on both sides of the Atlantic.

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