An often stated reason was to get China to put pressure on North Vietnam to come to the negotiating table to allow the US to exit the war, hopefully keeping South Vietnam independent. The bombing and invasion of Cambodia can be seen as trying to put further pressure on the North. Of course while the North came to the table, South Vietnam could not be saved.

Another possible reason was to try and pull China further away from the USSR after the Sino-Soviet Split. In this regard there was more long term success. While China was never drawn into the western alliance and tensions remained they're connected to the USSR declined, especially as factories were set up to produce western goods.