In 1274 and 1281, the Mongols attempted to conquer Japan, but, in both cases, the invasions were unsuccessful because of massive typhoons. These severe weather events destroyed most of the invading ships or forced them to return home. What is the relationship between the two sentences in this excerpt?

The relationship between the two sentences in the excerpt is that the second sentece is explaining the reason why the events of the first sentece ocurred.

In the first sentence, a fact is introduced: the Mongols tried to invade Japan twice, but typhoons impeded them from doing so both times. The second sentence clarifies why: the severe weather produced by the typhoons destroyed most of the Mongol fleet, which forced them to retreat.

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