Sorry if this was discussed already.
What are consequences of Portugal joining World War 2?
How about Portugal joining on the side of the Axis?
How about them joining the side of Allies (maybe Japanese invade Macau, does this also mean a Spanish involvement in the war?)
The Japanese did invade Portugese East Timor and Portugal did not get involved. I would say about the only way to get Portugal actively involved in WWII would be for Portugal itself to be attacked by one side or the other.
A possibility for Portugal siding with the Axis during WW2 would be the failed assassination attempt against Salazar in 1937 to succeed and someone even-more right-winger to replace him, but capable enough to avoid and defeat military rebelions from the old republican right and left (like happened with Salzar in OTL). This would require some kind of apparent moderation (pre-WW2) to avoid sparking even more rebellious (for example from the old republican right).
For Portugal to side with the allies, it would take some kind of attack from the European members of the Axis.
The Japanese did invade Portugese East Timor and Portugal did not get involved. I would say about the only way to get Portugal actively involved in WWII would be for Portugal itself to be attacked by one side or the other.