It is two decades since the euro was last trading below $1.00 (£0.84) against the US dollar. Now the single currency is once again teetering on the brink of parity.This, though, is not just a story of euro weakness, it is also a tale of dollar strength. Investors traditionally flock to the US currency in times of uncertainty and there are plenty of reasons – war, inflation and the prospect of fresh measures in China to combat the Omicron variant of Covid-19 – for them to be jittery.