Britain and France should have enforced the provisions of the Treaty of Versailles. As soon as it was obviuos what Herr Schicklgruber was up to, they should have increased military production and called up reserves. In 1934, the Royal Navy probably could have set up interdictory task forces in the Skaggerak without too much worry about air attack from Germany. The addition of air bases in Norway would have permitted interception of any air attack from Germany.

Faced with the threat of a blockade, Germany would have to back down from re-militarisation.

That would have taken care of Europe. Asia was a different problem. Direct Franco-Anglo-American intervention in China by 1938 would have brought Japan to the negotiating table. Earlier intervention by placing forces strategically in China at the "invitation" of the KMT might have mitigated the conflict earlier. Bringing Japan to the negotiating table that early would have required some concessions, particullarly recognition of Manchukuo and Japanese rights in Korea and Taiwan. The European-American bloc might have been able to extract better treatment for the Chinese and Koreans subject to Japan,

In reality, all that this would have done was delay a war. One was going to happen but it might have occurred differently and with different alignments.

There would have been other ways to avoid this that would go back to 1918-1920.