to hold the highest office = to be in the top position Obama currently holds the highest office in the US of A. to be in harm's way = to be in the path of oncoming danger Soldiers are often in harm's way when they are in enemy territory.

"In harm's way" means in danger, like soldiers in battle. "Holding the highest office" means having the most power because you have the top position in a department. The passage explains that soldiers are happy because another soldier holds the highest office in the department of defense. They know that someone with experience controls the US Defense and the military.

"In harm's way" means in a place or situation that is dangerous. They are in danger of being harmed. "Hold the highest office" means to be the person with the highest rank or position. The highest office in the US Department of Defense is the Secretary of Defense. This meaning of secretary is like "minister" in other governments, not like a secretary in an office.