The American military has three primary roles: 1) to protect American interests around the world; 2) to assist nationally and internationally in times of natural disaster; and 3) to intervene in foreign genocidal conflicts and wars of aggression.
The American military can no longer exist as an instrument of American political will unless formally engaged by the ruling government or a treaty organization. The era of America embracing a "manifest destiny" mindset as rationale for being involved in foreign national development is over.
American intervention should no longer be used even as a threat in the modern world. Let's face it, we no longer follow through anymore so the threat is rarely a deterrent anyway.
American interests need to be clearly defined as those with business, social or individual components. Humane and appropriate economic treatment of corporations and citizens that are operating or living within a foreign land need to be adhered too. If any attempt is made to nationalize businesses, inflict inequitable taxation or operating standards, or inflict personal injury upon individuals then the military has a right to defend it's institutions and citizens. Intervention however,can only occur when these injuries are incurred outside the legal parameters of the country in which they are are incurred. Americans though, need to be cognizant of the threats inherent wherever they do business and if they choose to enter a specifically dangerous area then they assume that risk as part of their normal freedom of operations.
When natural disasters occur the American military as well as the military units of other countries should be made available in terms of manpower and resources to incur a safe zone within the affected area and to support relief efforts.
Intervention is warranted whenever tyrannical foreign leadership is utilizing genocidal tactics as instruments of control and power. A prime mandate in the 21st century for the American military is to uphold human rights and protect civilians from persecution by their own government. The world has to come to realize that the number one offense that any government can inflict on it's citizens is death. Government has inherently an obligation to create a peaceful environment for it's citizens and orchestrated extermination of any portion of that population will meet with immediate intrusion by military forces. The style of government is inconsequential.
Intervention by the American military during periods of national uprising or civil disobedience is not warranted at the outset of any conflict. Civil uprising and governments defending their position is a natural part of nationalistic development and America has no role in disputes of that nature. However, that being said, if conflicts continue with no resolution in sight, then America can be invited to participate by the ruling government. It is not America's role to pick sides in civil conflicts. Let's face it, if foreign countries had decided to support the south during the American Civil War then the results may have been very different and our nations sovereignty and right of self-determination would have been severely undermined and we likely would have developed very differently.
Any act of aggressive attack by a foreign country by another nation shall meet with American intervention as well. War of acquisition is no longer a valid form of conflict in the modern world and has to be opposed by strength. Historical boundaries are still quite often in conflict and on occasion governments have tried to re-establish historical boundaries. This is not a valid reason for military action against a neighbor. The era of extending national boundaries and influence through international territorial expansion is a thing of the past.
In the future, it is is highly probable that wars of acquisition will occur in order to gain access to the world's most valuable natural resources (oil, water and precious metals). When conflicts of this nature arise America has the right to be involved as a means of protecting the assets of foreign countries and establishing a global rule or territory and ownership. America will also have the right to intervention if any valuable resource is being withheld as a means of the foreign country establishing some type of political advantage. The military however, cannot be involved if resources are being sold elsewhere as this would have to be addressed in the private and public sectors.
The bottom-line for the 21st century is that our military has a definitive role that can be utilized in a more specific roles. The end result will be the ability for America to adopt a more consistent role in the deployment of it's military and one that will be welcome and appreciated when it is utilized. By managing the military in this manner it will give America a favored position in the world as the military becomes seen as a beneficial presence globally rather than an unwelcome intruder.