The Constitution gives power to Congress to declare war as a check on Presidential power. However, he inherits the authority to use troops if he encounters an immediate threat to the United States and Congress does not have the opportunity or the time to declare war. So, how does one amend the parameters to limit him? What sort of threats is so immediate and dangerous that the President should find the need to use force?
Commander-in-chief in recent history
If one looks at recent history when America has sent troops to the conflict, especially during the 20th and the 21st centuries. It has been done with him acting as the commander in chief without a formal declaration of war from the Congress. Examples include: Korea (1950 - 1953), Vietnam( 1955-1975), Iraq(1990-91) and (2003-11).
After Vietnam, Congress tried to put the brakes on passing the war power resolution, which required the President of the United States to get authorization to use troops within 60 days of their commitment. He is advised to bring them back in case the authorization fails. However, in practice, Congress almost always formally approves his power to use force.
Treaties and agreements
As a diplomatic stance, the constitution officially allows the President to make treaties, receive and appoint ambassadors and ministers.
The President has developed the power to negotiate Executive Agreements as well.
The most important difference between an Executive Agreement and a Treaty is that an agreement doesn't need to be ratified by two-thirds of the Senate and is valid with only a majority vote in both houses.
The President is the CEO of the United States
Although it isn't mentioned in the constitution, the President is the chief executive officer or the CEO of the United States. Formally it is in the faithfully executed clause of the Presidential oath of office, but more importantly comes from his power to appoint judges, ambassadors, and other ministers.
The President chooses administrative heads that agree with his policies. His appointments shape political agenda, where he assumes the inherent power to direct the ministers and their agencies on how to implement laws. Enabling him to address the way the government acts.
The President can attempt to set the legislative agenda by making recommendations for laws he'd like to see passed. This is called the legislative initiative and in practice usually involves the executive branch drafting legislation to the Congress which is refined to make it pass. This happens with big agenda items like the affordable care act, also known as the Obama care act or the Dodd-Frank act which despite being named after its two congressional sponsors was written with input from the white house. One should also note that Congress often willingly gives power to the president of the United States to avoid responsibilities on traditional policies.
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Another legislative authority and probably the most important one:
He can give Executive orders. These are Presidential directives or rules that have the force of law. Executive orders can be overturned by actual congressional lawmaking or by the decision of the supreme court. Executive orders allow the President to circumvent the legislative process and act unilaterally. Ideally, the President and Congress work together.
Executive orders may not be as durable as the laws passed through normal channels if the next President in office disagrees with the order of the United States president. He or she can get rid of them just as easily as his/her Predecessor put them a place.
The President can impound funds the Congress has appointed to individual programs or projects if he doesn't want them implemented.
The ultimate inherent power of the President is Executive Privilege. This is the President's ability to keep information secret by claiming that it is too important to be revealed, usually for reasons of national security. The Executive Privilege can, however, be overturned by a court order.