A system of checks and balances prevents one branch of government from becoming too powerful. The Constitution provides the framework by which each of the three branches of government--executive, legislative and judicial--can resist encroachments and check the other two branches.
For example, while congress makes laws, the president has veto power, and the judicial branch can rule on which laws Congress intended to apply to any given case, as well as on the constitutionality of the laws. The system of checks and balances is also at the foundation for important appointments to executive departments and federal courts. The president appoints federal judges and executive department heads, but the Senate, part of the legislative branch, considers and votes on these appointments, and can block the executive's nominations. Under the Constitution, the sole power to declare war and to regulate, fund and maintain the armed forces resides with Congress, while the president is the civilian Commander in Chief of the Army and Navy of the United States. Even though the president is not authorized to declare war, it is generally understood that the president has the authority to command the armed forces to take military action in the event of a crisis.