There are two potential use cases for conventional forces (this is likely similar to what Gates is thinking as he proposes cuts/restructuring):
- Against a peer competitor. Simply, a country like Russia or China that has a force that can deploy top of the line technology and significant manpower.
- Against a proxy of a peer competitor, that is supplied with top of the line weapon systems but non-nuclear.
There are significant caveats with both scenarios.
Wars with Peer Competitors
Peer competitors are, nearly by definition, nuclear powers. We have never had a "hot" conventional conflict with a country armed with nuclear weapons. Simply, a war of that type (the historian Martin Van Creveld makes the case for this better than me), would risk a global nuclear catastrophe. This makes a conventional conflict with a peer competitor merely a fantasy. In reality, the role of conventional forces in this type of conflict, is as a deployable trip wire. This means that the conventional forces we have must be able to deploy in great enough mass to edges of what we consider inviolate territory -- as in the loss or damage to these forces would force a nuclear war.
Wars with Peer Armed Proxies
In this class of conflict, the peer competitor arms a proxy nation with top of the line weapons. Unlike during the Cold War, the arming of these nations, isn't a politically driven process. It is economic. Countries can only get as many weapons as they are able to afford (considerably less than during the peak of the Cold War). Therefore, conflicts with countries of this type, would be against a competitive force that is relatively weakly armed in terms of high end weapons. Additionally, since we are all economically interconnected now, it's unclear how many nations would actually fall into this group. The number is likely already very small and shrinking.
What this means
In the past years, we have spent $400 billion plus on conventional weapons systems and personnel per year. Given the budgetary pressures we are currently facing, this amount could be reduced by half or more without sacrificing any measure of security given the threat we could face today or in the future. If a more specific number is needed, perhaps a measure of spending that corresponds to the defense budgets of both China and Russia combined would be a good starting point.
NOTE: In contrast to the above, spending on special operations, resilient community technologies/development, etc. should be increasing. The number of scenarios for this type of capability are multiplying.