When did Uruguay join World War 2?

It didn't, Uruguay remained neutral during WWII. However, when one nazi ship reached Uruguayan coasts being followed by the UK army asking for parts to repair its ship nobody help them and the ship was sunk by its captain in order to avoid the UK to have the nazi ship. Most of the parts of the nazi ship has been taken back from the bottom of the estuary Río de la Plata (River Plate) and are shown in Uruguayan museums nowadays.