The New Left taking it over in the 70s from the liberals was what did that.
To reverse that, you'd need the New Left to fail or form its own party. Or the Republicans could cool it on the anti-Communism and become more libertarianish.
I don't really think you can call either party a party of war or peace. The Democrats may have been against the Iraq War, but the Republicans shot down the concept of a Syrian War three years back, and in both cases, there were exceptions.The GOP largely was opposed to intervention in the Balkans. Both parties supported the war in Afghanistan to the hilt, as well as Gulf War I. Neither party was in favor of direct intervention in the Israeli-Hezbollah war, but both parties were largely fine with aiding Israel.
So this is kind of an odd question to start with.