The president was making an annual speech to commemorate the 72nd anniversary of the nation's liberation from Japanese military rule that began in 1910 and ended in 1945.
"The government, putting everything on the line, will block war by all means," Moon said.
Previous presidents before Moon, who took office in May, have traditionally made North Korea the core of their speeches, mainly focusing on policy to engage Pyongyang.
Moon also urged the North to come to the dialogue table, saying sanctions against Pyongyang aim only draw it out to talks.
"I call upon the North Korean government: without international cooperation and co-existence economic development is impossible," said Moon. "If you continue on this path there will only be international isolation and a dark future."