Defensive and aggressive wars

Modern imperialist war has an inherent contradiction, which often explodes in revolutionary fury at the end of the conflict. The imperialist ruling class often requires mass mobilisations, the mass conscription of working people into its army and heavy taxation for a war that benefits only itself. To overcome this contradiction, the imperialist rulers have to lie to their own people to gain mass support. The primary lie is: “We did not want this war. It was unavoidable, forced upon us by enemy aggression. We are only defending our nation.”
While the question of who was the aggressor may help in disproving government lies and understanding the politics of a war, war politics can’t be reduced to who is the aggressor or the most aggressive, and who is the defender. The most primitive level of this question is, “Who fired the first shot?” When war begins, all sides attempt to pin the label of aggressor on the other side, manoeuvring to make the antagonist appear to be firing the first shot. The purpose is to manipulate public opinion into believing the claim of a war for self-defence.
The Vietnam War, for example, began with such an incident in August 1964, when the United States charged that two US naval vessels were fired upon off the coast of North Vietnam in the Gulf of Tonkin. The US House of Representatives voted 416-0 for the Gulf of Tonkin Resolution, which gave the government the authority to take any actions it deemed necessary to “defend South East Asia from communist aggression”. Five years later, the Pentagon Papers revealed that President Johnson had written this resolution months before, and had waited to introduce it until Washington could claim that it had been attacked. The Pentagon Papers further documented that the two US warships were off the coast of North Vietnam on spying and kidnapping missions. It is not clear if the North Vietnamese in fact fired on the ships. Even if they did, that fact tells us very little about what caused those shots to be fired or the policies behind the so-called “aggression”.
A more sophisticated orchestration to convince the US public of the defensive nature of Washington’s war was the plan for the launching of the US air war against North Vietnam in February 1965. The decision to bomb had been made three months earlier by Johnson and his advisers. They ordered the Joint Chiefs of Staff to devise a strategic plan, which they called “provocative engagement”, i.e., to stage repeated acts of provocation, covert raids against North Vietnam in the expectation that the Vietnamese would eventually retaliate. When, after months of this operation, the Vietnamese did attack a US base at Pleiku, in South Vietnam, the Pentagon began the previously planned air war, claiming it was a defence of South Vietnam from “communist aggression”.
Despite Washington’s manoeuvring for a war it wanted, had the Vietnamese been the aggressor, the underlying politics of the war would not have changed: it was a just war of national liberation on the Vietnamese part, and of imperialist conquest on the part of the US — no matter who fired first. If the political goal of the “aggressor” is just, we support the aggressor. (Marxists might in fact support an aggressive policy that results in war to gain a goal like national liberation.)
In sum, the issue of whether one side was the aggressor may tell you something about whether or not that side is lying about the claimed defensive nature of its war and may provide indications of what its real objectives are, but it cannot tell you the most important questions about the political dynamics of the war.