He was a solid president, but he made 2 major mistakes.
Iran coup detat- Overthrew the democratic Iranian leader and installed the Shah who went on to torture and kill many people. Generally pissed Iran off and then they had the Revolution and hated the US since (extreme sparknotes)
Helped build the military industrial complex (fancy term for governments intermingling with corporations for military equipment). He later warned of it, but his administration also helped build it.