Who is the only person in the us who has the power do declare war?

Pursuant to Article I of the United States Constitution only Congress may declare war. As a matter of practice, however, Congress no longer declares war but instead issues war powers resolutions under the War Powers Act, which resolutions are documents that sort of provide parameters to the President on the extent of his authority to use the military. Further, in practice, the President tends to stretch these resolutions beyond their express terms, or sometimes ignores them all-together and initiates military actions (like Libya, for example) without congressional authorization under his Article II powers to defend the country's national interests.