all the countries backing us look pretty stupid right now and are prob looking at uk and saying... boy who cried wolf once too often now.
They're all still backing the UK. Obviously Russia is the only likely candidate for using a Russian made nerve agent in a manner consistent with Russian assassinations against a Russian target.
The only alternative would be someone like the US doing it to frame Russia, but they were so slow to respond and initially were so half hearted about it that they don't look likely instigators. I wouldn't put it past the US doing that (or Israel if they had any motive at all) but US failed to capitalise on the situation. There aren't many states capable of pulling this off.
Seems a little silly and Sisuesque cukoo conspiracy theory to think it was anyone other than Russia.
