Final answer:
The constant fighting in Japan between 1281 and 1590 was a result of the crumbling of the shogun dynasty, which left the daimyos and samurai without a unifying leader. This led to power struggles and conflicts among them.
Step-by-step explanation:
The constant fighting in Japan between 1281 and 1590 was due to the fact that the shogun dynasty of Kamakura had crumbled after the unsuccessful Mongol invasions. Without a strong shogun to unify and govern the daimyos and samurai, they were free to pursue their own interests and ambitions, resulting in constant conflicts and power struggles.