Why did the articles of confederation assigned foreign affairs and the ability to declare war and peace to congress?

The Articles of Confederation and Perpetual Union was an agreement among the 13 original states of the United States of America that served as its first constitution.[1] It was approved, after much debate (between July 1776 and November 1777), by the Second Continental Congress on November 15, 1777, and sent to the states for ratification. The Articles of Confederation came into force on March 1, 1781, after being ratified by all 13 states. A guiding principle of the Articles was to preserve the independence and sovereignty of the states. The weak central government established by the Articles received only those powers which the former colonies had recognized as belonging to king and parliament.

the secretary of state, appointed by the president with the advice and consent of the senate, is the president's chief foreign affairs adviser. the secretary carries out the president's foreign policies through the state department and the foreign service of the united states.