It is a popular myth that India Pale Ales were made stronger than regular beers in order to assist them to survive the ardours of long sea travel. Export beers were more heavily hopped, but not necessarily stronger.
If you want proof, here it is, an IPA from 1914 at only 4.2%. This predates the impostion of wartime restrictions on drink supply and strength, so was a commerically available beer of the day.
English-Style India Pale Ale
ABV:
4.20%
Florence Brewery
London, United Kingdom