In hindsight, it is quite obvious that the USSR did, and at the time even, you would think that a reasonable person would see events happening and reach that conclusion, but it simply isn't the case!

First, lets look at the text from the ["Agreement of Mutual Assistance between the United Kingdom and Poland, August 25, 1939"](https://en.wikisource.org/wiki/Agreement_of_Mutual_Assistance_between_the_United_Kingdom_and_Poland-London_(1939)).

Although the phrase "Should one of the Contracting Parties become engaged in hostilities with a European Power in consequence of aggression by the latter against that Contracting Party, the other Contracting Party will at once give the Contracting Party engaged in hostilities all the support and assistance in its power" would seem to be straightforward, it really isn't! There was also the Secret Protocol which stated that "European Power" was little more than a euphemism for Germany, and that if another power invaded "the Contracting Parties will consult together on the measures to be taken in common."

So yeah, there was a big, glaring caveat, and while the invasion on Sept. 1st saw Britain fulfil what was a clearly stated obligation, on Sept. 17th, Britain was happy to stretch that language to the breaking point, no matter how much they might not have liked Soviet actions. And why wouldn't they? There was definitely belief that Soviet-German cooperation wouldn't last and they would eventually turn on each other, and a declaration of war would jeopardize that, forcing the two into closer cooperation. Poland was doomed, and Britain knew she had no chance to actually save her from this initial invasion, so what would be gained by adding another belligerent to the war? When Sir William Seeds, the British Ambassador in Moscow, was asked his opinion, he responded "I do not myself see what advantage war with the Soviet Union would be to us, though it would please me personally to declare it on Mr Molotov."

So anyways, on the morning of the 17th, the Polish Ambassador visited the Foreign Office on what he really already knew was a futile mission, understanding that the British Government would (publically at least) state they had considered 'other European power' to be Italy, even though Poland, and perhaps any reasonable person, would have seen the USSR as the next biggest threat. The Cabinet met the next day to consider the matter, and prefered to do an ostrich act than play the lion, deciding simply to formally protest Soviet actions to register their "horror and indignation", and maintaining "complete confidence that on the conclusion of the war Poland would be restored."

Poland of course protested, communicating to Lord Halifax, the Foreign Secretary, that "the Polish Government reserve the right to invoke the obligation of its allies arising out of the treaties now in force", but was succinctly rebuffed, which Halifax emphasising the British interpretation of the treaty, namely that they had no obligation to do anything other than consider the matter, meant "we are free to take our own decision and to decide whether to declare war on the USSR or not."

So within only a day or two, the Soviet action was a fait accompli and if anything, the British government now went on a PR campaign to try and justify its lack of action. In October, after Poland had fallen, Lord Halifax addressed the House of Lords to note:

It is perhaps, as a matter of historical interest, worth recalling that the action of the Soviet Government has been to advance the boundary to what was substantially the boundary recommended at the time of the Versailles Conference by the noble Marquess who used to lead the House, Lord Curzon, who was then Foreign Secretary.

And although not a member of the government, it is also worth mentioning Lloyd George's editorial in the Sunday Express entitled "What is Stalin Up To?" published in late September, which essentially justified the Soviet movement and pushed their own explanation that it was intended as a humanitarian action to protect the people who lived in eastern Poland (In the words of the Soviets, "The Soviet Government also cannot view with indifference the fact that kindred Ukrainian White Russian people, who live on Polish territory and who are at the mercy of fate, should be left defenceless"). When Ambassador Raczyński attempted to have a refutation published in the Times, he was rebuffed and had to arrange for private printing of it.

In the end of course, you the UK didn't necessarily make the wrong choice (for them. Poland definitely got screwed). I don't want to deal with counterfactuals, but the possibilities for how war would have progressed if the UK and France had entered open hostilities with the USSR certainly don't seem to improve the odds for the Allies. The travails of Poland through the war, and beyond, are really another topic, but suffice to say that the UK continued to ignore what was staring it in the face, and continue in its (public) belief that Poland would be restored, although in part we can say that it was a continuance of their "we need to not piss off the USSR" policy, and when it was clear they were mistaken, it was too late and there was nothing to be done anyways, leaving us with the sad irony of the war, that it was launched on the casus belli of maintaining Polish independence, and in the end, Poland would spend a half-century behind the Iron Curtain.

Kochanski, Halik. The Eagle Unbowed: Poland and the Poles in the Second World War Harvard University Press, 2012

Prazmowska, Anita. Britain, Poland and the Eastern Front, 1939 Cambridge University Press, 1987