why Religion and State don't mix

Confessions of a Confused Religious Zionist - written by Rabbi Shlomo Cradall is very enlightening piece.

religious Zionists in the army have difficulty choosing whether to follow their religious leaders or their military commanders...

"Dr. Matar also explained that, as a religious man, he could not accept orders that would force him to choose between his fundamental religious beliefs regarding the Torah commandments to settle the Land of Israel and his loyalty to the state and the army."

Here is the full article.