Just to answer my question in the previous post...
Thanks to Elaine Elinson's letter in the guardian for the information, apparently the last president of the US to use God as an explanation for his decisions was William McKinley during the Spanish-American war.
"I went down on my knees and prayed Almighty God for light and guidance more than one night. And one night late it came to me this way... that there was nothing left for us to do but to take them all and to educate the Filipinos and uplift them and civilise and Christianise them."
McKinley invaded the Philippines resulting in more than 600,000 natives dead.