What if John of Gaunt became King of Castile in 1374?

In 1372, John of Gaunt began claiming the Castilian throne due to his marriage to a daughter of the reigning king, and by 1373, he had a plan to invade the country and depose the incumbent Henry II. In 1374, he made a pact with Charles the Bad of Navarre to allow his armies passage through the country. However, only a few days after this agreement was reached, he returned to England and abandoned the plan. How would the history of Europe be different if this plan had succeeded?
 
I'd like to know how John went from a tactical failure (see his French campaign...) to a man who defeats three to one odds against him to beat Henry II. Also, the Castilians are not going to accept him as King, merely a consort to their Queen.
 
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