As everyone knows, the Japanese went south, conquering most of the Pacific and attacking Pearl Harbour. But why didn't the Japanese wait until Hitler went on the eastern campaign (attacking the Soviet Union in 1941) and then attack the SU from their side? They must have known Hitler's plans of going east, since minimal diplomatic relations between the two existed? I know the two were allies more on paper then in reality, but still. Also I am aware of various Soviet Union and Japanese border skirmishes, like Khalkhin Got and etc.
Japan could expand in two directions: west and northwest or south. First they wanted to go west (China) and northwest (Siberia). China wasn't a problem, but going northwest was. The soviet army was much better than the Japanese expected. They were soundly defeated in the battles of Khalkin Gol. One of the commanders on the USSR side was Georgy Zhukov.
Those battles were extended border clashes, but more than enough to show the Japanese army it wouldn't be a walk over. So they stopped there and then. One of the reasons likely was that Siberia had a lot of minerals, but you need to extract them. That's pretty hard, and would take many years.
As to the Axis treaty: it was a very nominal treaty. Just about nothing was done in cooperation. All they shared were some signatures on paper. There was no coordination, conflicting interests and very little else. The Japanese attack on Pearl Harbour was as much as a surprise to Hitler as it was to Roosevelt. A bigger surprise for Hitler, because the Americans could read the Japanese code.
Going south was much more profitable for the Japanese. The oilfields in the Dutch Indies were huge and in production. Malaysian and Thai rubber were for the taking. All they had to do was knock out the US fleet in Pearl Harbor.
Yamamoto warned against it, but not so wise heads prevailed. With hindsight you can win any war, this is a clear example. It's even worse for the Axis. Richard Sorge was a Soviet spy in Japan. He could conclusively prove to Stalin that Japan wasn't going to attack in Siberia, and he could safely withdraw the Siberian troops and use them for the defense of Moscow. That was even worse than not cooperating and not supporting each other.
Summary: Japan couldn't wait
So the Japanese were all fired up to fight. They decided to split the Soviet Union with the Germans, right? Met with the Germans in November 25, 1936 and were ready to fight.
The smart thing would have been to wait until the Germans were ready. However, Germany still had to annex eastern Europe first, and the Japanese Army was unwilling to wait a few years. It was a function of the Japanese power structure; they had to fight.
The Japanese emperor said that Japan received an elementary school on Hassan and a secondary school on Khalkhin Gol. For this reason, the Japanese were in no hurry to receive a "higher education". Japan decided to start a war against the USSR if it was occupied by Leningrad and Moscow. Guaranteed victory.