There are many problems with looking simply on the issue of legality. First of all, international law is not domestic law. It is a framework without an overarching "sovereign", so "enforcement" of international law is different than in the domestic context. International law is also based, at least in part, on norms. But norms change – thus changing what the law means.
Before World War I, unrestricted submarine warfare was considered a violation of international law. The US even used Germany’s submarine warfare to declare war on Berlin. By World War II, however, the practice was so widespread that during the Nuremberg trials, the charges against German Admiral Karl Dönitz for ordering unrestricted submarine warfare were dropped, even though the activity was a clear violation of the Second London Naval Treaty of 1936.
In which case, one of the most important criteria of all is the national interest. Do the actions of a government advance its (and its citizens’) interest? I don’t mean to create too dichotomous a notion – the best possible system of international law is clearly in Britain’s interest most of the time, and reflects our self-conception as a civilised people who operate under laws, not only force. But even an illegal act can advance the national interest.
Just look at the Netherlands. Though their version of the Chilcot inquiry has said that Dutch involvement in the war was illegal, it was clearly in the country’s interest to join the US – it brought them closer to the US; meant their foreign minister could become NATO Secretary-General; helped them upgrade their military, and more besides.
Naturally, this has to be weighed against the cost of the war: the countless deaths, the instability in Middle East and so on. But sidestepping the discussion and judging the Netherlands’ participation solely on whether the war was legal or not seems peculiar.
The same could be said here. Views will obviously differ about what is most important – legality, legitimacy, national interests or global values – but let’s have that debate, rather than closing it off by focusing only on legality.