One weakness of the articles of confederation was that congress could not a. declare war. b. impose taxes. c. set up a postal system. d. create an army and navy.

The Articles of Confederation did not have the power to tax.  This among some other significant weaknesses were the reasons the Articles of Confederation failed and The Constitution exists.  The inability to tax the states or the population left the Congress broke.  The only source of funds they had was to borrow money from the states and these loans never came close to the financial needs of Congress.  They had very expensive tasks such as creating an army and navy, setting up a postal system, issuing a common currency, as well as sending and receiving ambassadors.  Without the power to collect tax Congress did not have the money to exercise all of it's powers and provide for the states, therefore it did not win the confidence of the states.  Most did not take it seriously.

The Articles of Confederation were intended to be "a firm league of friendship" between the states, however states would still maintain their sovereignty, freedom and independence.  This may of worked in theory, but inability to tax wasn't the only failure of the Articles of Confederation.  Another problem was that Congress did not have the power to regulate trade between the states, this meant that commerce had no common management. Additionally,  The Congress had no power to enforce or make states obey the Articles of Confederation, so if a state refused to follow the rules Congress was basically powerless to make them do so. A final failure was the requirement for all 13 states to approve of any changes, or amendments, to the Articles of Confederation.  This meant that all votes for congressional changes had to be unanimous or they failed.  All of these failures combined ultimately lead to the adoption of The U.S. Constitution that we still live by today.