So it was something of a surprise to see the US president, this week, apparently riding to the rescue of Silicon Valley.

He instructed his top US trade official to start an investigation into France's plans to raise a 3 per cent tax against the revenue of large international technology companies.

Emmanuel Macron has championed this tax as a way of getting these companies to pay what he says is their fair share in France, rather than in offshore tax havens.

The companies themselves say they're being unfairly targeted.

And warn that they could end up paying tax on the same transaction twice in two different countries.

The US and France now have 12 to 18 months to try and come up with some kind of negotiated settlement.

The US could, for example, issue tax credits against the additional levies being paid by American companies in France.

But the US Treasury is unlikely to want to give up on its own lucrative revenue streams.

If there is no agreement reached in that time, there is the possibility that the US could raise retaliatory tariffs against French goods.

Sources tell me that officials have been looking at ways to target French wine, for example, or cars.

For the US technology companies themselves this would be the worst of all worlds.

Not only would it make it less likely that France would back down, but it could even spark a US-France trade war, which would harm business for everybody.