In 1950, the USSR boycotted the UN over the Chinese Nationalists, rather than the Chinese Communists being represented there. During their absence, North Korea invaded the South and the UN voted on whether to invade.
Had USSR been present, they could have vetoed the resolution to go to war.
Was the ambassador simply unable to get Stalin on the phone and ask for permission to break the boycott to perform the veto?
OR
Did Stalin want the war to go on as an opportunity to defeat the west?
The North Koreans invaded at dawn on June 25th 1950, which is the middle of the afternoon June 24th at UN HQ in NYC. Within 5 hours news agencies had picked it up. 24 hours after the invasion, resolution 82 was introduced and passed. Security Council Resolution 82 demanded hostilities end, and the North Koreans withdraw.
Two days later, Security Council Resolution 83 was passed authorizing UN member militaries to assist South Korea.
Their boycott didn't change the situation much. Prior to their UN boycott on January 13th, 1950, the Soviets abstained on most on Security Council resolutions. Of the 78 resolutions prior to the boycott, they abstained or were absent for 43 of them. This is quite unlike the other security council members at the time (except for Ukraine, a Soviet puppet at the time, which also abstained a lot). This suggests they didn't take the UN seriously, didn't see unprecedented military intervention as a possibility, and would be caught on the wrong foot when it happened.
In this day and age of instant world-wide communication and fast jet travel, it's difficult to understand just how slow communications and travel were in 1950. No internet (of course), but even making a long distance phone call was a nightmare. Making one from the US to the Soviet Union was even more difficult.
Jet airliners were still a few years off, if you need to get your UN representative back to New York that means multiple flights on a slow, propeller driven aircraft.
Now imagine the situation for the Soviets. They were well aware the North Koreans were going to invade, they assisted in the planning. And they were concerned the US might intervene. But they probably weren't expecting the untested UN Security Council to take military action. On top of that, Yakov Malik, their ambassador to the UN, had spectacularly walked out of the UN in protest of their seating the representative of Nationalist China over Communist China, and declared a Soviet boycott.
The first the Soviets would learn about a UN intervention would probably be as Security Council Resolution 82 is being passed. The clock is now ticking.
This information has to be encrypted, typed up, sent from NYC to Moscow across multiple communication lines, received, typed up, copied, and sent up the chain of command. Then Moscow has to debate what to do about it.
Is this resolution even a concern? So what if the UN says the war is illegal, what are they going to do about it? The UN is young and untested. It's predecessor, the League Of Nations, was spectacularly ineffective in stopping aggression prior to WWII. The Soviets had a habit of successfully running roughshod over international treaties.
Then, if they do decide it's a concern, they have to debate whether that possibility is worth ending their UN boycott and possibly angering Communist China.
If they decide to return their ambassador to the UN, it has to be typed up, sent down the chain of command, encrypted, transmitted back to NY over multiple communications channels, received, decrypted, typed up, given to the relevant people, who then have to decide what to do about it.
Once that's all done, Yakov Malik has to get to UN HQ from wherever he was at the time. Considering the boycott began in January, he might not even have been in NYC or even the US at the time.
All in two days.
This answer is quite speculative, knowing the actual deliberations within Soviet High Command and the location of Yakov Malik at the time would lend it some more weight. But it's quite useful to understanding the scenario the Soviets faced and how difficult and slow international politics was in 1950. It's not like today where people would hear the news within minutes, decisions would be made within hours, with frantic phone calls back and forth, and ambassadors hopping onto jets to be in NYC in 8 hours.
The US, with the UN HQ right in their backyard and driving the whole process, had a serious home field advantage.
The USSR boycott wasn't simply a bout of pointless petulance. It was likely felt in the USSR that any action from the Security Council required their concurrence whether or not they were there for the vote, and thus their boycott would prevent anything from getting done (eventually forcing acceptance of their position on the China issue).
The veto power itself is essentially implicit, rather than explicit in the UN charter. All it says is that "the concurring votes of the permanent members" are required on "substantive" decisions. However, a member can abstain from a vote, as a way of allowing something they disagree with to pass without going on record as voting for it. So it was debatable what not showing up for a vote at all meant. Many legal scholars felt that all such members had to be present. However, there was some precedent for treating an absence as a non-blocking abstention.
In the event, the US and all the other SC members proceeded to hold votes without the USSR, and then act as if unanimous consent of those present was good enough, over the protests of the USSR. Rather than potentially allow even more such votes go against them, the USSR effectively conceded the point by ceasing their boycott.
