What's a good name for a military rank that's above a Marshal but below a Supreme Commander?

So, for the context, in my world, I have three branches of the military, each with a general staff. For each general staff, there is a Supreme Commander, who is also the chairman of the joint chiefs of staff, and a vice chairman. <br><br>The Supreme Commander is a lifetime position, that is only changed when the Supreme Commander dies, retires, or is removed without cause. The general staff for each branch elects their Supreme Commander without any interference from the Head of the Federal Republic.<br><br>The second position, which I'm trying to find a name for, is a war time rank, that is only used in a declared war. This position would be responsible for coordinating all branches of the military in campaigns and theatre of operations. It would be occupied by a member of each general staff, that is elected by the Supreme Commander. <br><br>I see no reason for this position to have a formal title of its own, and it would be outranked by the Supreme Commander, but I am unsure of what would be a good title for a military officer that outranks a Marshal but would be below a Supreme Commander.<br><br>To be more specific, I'm looking for a title that is not already used in any national military.