First things first, Stop the War was set up to oppose the war drive against Afghanistan in the immediate aftermath of the September 11th attacks. Building on the organisation that came together to protest the bombing of Serbia in 1999, it put on flesh in the demonstrations leading up to and during the first phase of the Iraq War. It has also been very clear the Coalition's strategy is about building anti-war public opinion against the British government because, well, it's a Britain-based outfit. It aims to change British foreign policy by putting pressure on its democratic institutions via mass mobilisation, civil disobedience, influencing MPs, making the case against military adventurism, and so on. Furthermore, as even my cat knows, Britain is part of a web of alliances and strategic military partnerships. It has particularly close ties to the United States, and in the Middle East, Israel and Saudi Arabia. Wherever these states are involved in military activity, it is usually with the tacit or practical support of the British government. The 2006 Lebanon War, the 2009 and 2014 war on Gaza, and current commission of war crimes by Saudi Arabia in Yemen incur the opposition of Stop the War because of our complicity. For Israel, UK and US companies have happily furnished them with armaments to drop on largely defenceless populations. For Saudi Arabia, not only are we providing ordinance but also "military advisors", thereby exposing British military personnel to possible war crimes charges in the future.
With a Labour Party forever divided on questions of war and peace, someone has to take up the cudgels of making the case against Britain's wars.
Oh, but what about Russia and Assad? Sure, there are tankie nostalgics in Stop the War, but backward glances of this kind are very much a minority interest. The reason why the coalition doesn't protest against Russian militarism is a political calculation: what would such actions achieve? In the first place, picketing the Russian embassy is unlikely to change the minds of the Kremlin clique. With its reputation mud in most NATO countries, the kind of actions Stop the War undertake aren't going to have an effect. Whereas, say, a big march on the Saudi Arabian embassy has the potential of giving our government pause. Second, and most obvious - too obvious for our Boris Johnsons - if Stop the War begin agitating against Putin, that contributes to the case for war. Imagine, if half a million on the streets gives a British government jitters over its support for the latest US action, then the same number protesting the bombing of an aid caravan on Aleppo's approaches might encourage them in its Syrian no fly zone idiocy. Having Stop the War co-opted for a war drive kind of defeats their purpose.
I don't particularly like Stop the War's politics, but it is what it is. Instead of griping, there is nothing stopping Boris Johnson and his Progress cheerleaders organising their own gathering in Kensington Palace Gardens if they felt so strongly about "doing something". But they won't, which makes their criticisms of Stop the War sound like hollow point-scoring.