You don't know anything. There was no United Nations. There was the League of Nations, but whatever you called it, if the individual members collectively had no stomach for a fight, what good is it? The US did not want to get involved in another European conflict, of which there had been an endless string since before there was a US. World War I was just the latest. Britain and France did not have the stomach for another war, they were trying to get back on their feet after the ruinous war they had just been through, and then the worldwide depression of the 1930s. When Hitler violated the Treaty of Versailles that ended WW I (and sowed the seeds of German discontent that led to WW II) and started to build up the military he wasn't allowed to have, nothing was done about it by the British and French and others except feeble protest. When Hitler began making moves to take territory to the east, on the grounds that they were only taking back German territory, or acting to protect the large numbers of German-speaking people in those territories (Sudetenland and Danzig, and the bloodless takeover/absorbtion of Austria), under threat of force, the French did little. The British did little more, making feeble attempts to 'negotiate' a solution. Neville Chamberlain met with Hitler twice, and the 'solution' was to do nothing to stop Hitler from taking over the Sudetenland in Czechoslovakia in exchange for Hitler's promise on a piece of paper that he would be seeking no additional territory. Chamberlain famously waved that worthless piece of paper in the air on his arrival back in England and proudly boasted that they had secured 'peace in our time', while Hitler just continued on with what he intended all along, to take nearly everything to the east, to have the 'lebensraum' - the free space to live - that he wanted for his dream empire. Of course those plans were kept to himself and his Nazi leaders, as was the real purpose of his actions against Jews and other 'untermenschen' like the Slavs and Romany and the disabled. The sub-humans not deserving of life in his Germany. A lot of that was hidden from the Allied powers.
When Hitler didn't stop pushing for more, when he set his sights on Poland next, the British and French warned him that it would be war this time, but with them doing nothing but issue warnings up to this point, Hitler paid little attention to it. When he invaded Poland on September 1, 1939, both countries made good on their threats, finally, and declared war on Germany. But both were ill-equipped for war and could do nothing about Poland. Hitler had already made a secret deal with Russia that he was no threat to Russia, and so Russia stayed out of it and didn't contest for Poland.
And the rest that followed you can do the research on. There is much more complexity, MUCH more, to what I have merely touched upon. There was Italy, and there was Spain. People were preoccupied with their own troubles and did not want another fight with Germany, and that's why little was done but talk.