The Japanese did not go into Siberia because they had already tried that and the Soviets
handed them their ass on a plate. Nomonhan was a hard lesson and they had no reason
to expect a different result if they tried again, the IJA simply weren't equipped to face the
Red Army on an equal footing, regardless of the German invasion, and the IJN would be
of no use in a land war. The loss at Nomonhan was one of the reasons that the Imperial
command started looking to the South for resources, and any move into the European-US
zone of control was going to involve conflict with the US.
An attack on solely European Colonial possessions only combined with a scrupulous avoidance
of US targets in the Philipines would require a very careful use and placement of IJN carrier
groups. Basically you'd need to have them placed to give support when needed, but also be
placed to intercept any direct US response, if one came. An attack on solely colonial targets
and avoidance of US territories would put the US government in a very difficult position,
especially as the hardcore of the organised isolationists were almost fanatically anti-British,
so intervention would be a hard-sell. This would probably lead to a political delay which,
combined with the logistical and purely temporal realities of the USN sortieing from Pearl,
would give the Imperial forces a breathing period of at least several weeks, possibly as
much as six months.
However none of this means that the US would not become involved as the realities of already
existing US involvement in the Atlantic submarine war, and the depth of materiel support and
financial involvement, meant that the US was going to be directly involved at some point.
The Pearl Harbor attack served to advance the timeline.