As always, you need to look at the context. Here's what the sentence means, with some text from the preceding paragraph added and the clauses rearranged and annotated for ease of understanding:
Nervous investors turned to the safety of gold: signals from Washington were the main driver. The signals were that [Washington] was prepared to take further action against the Syrian government if [Washington] thought [Syria] was using chemical weapons.
Looking at it the original text, I have highlighted the main clause: we can see that was should be were in order to agree with signals:
Signals from Washington that it was prepared to take further action against the Syrian government if it thought it was using chemical weapons were the main driver, with uncertainty about forthcoming elections in France also simmering