Panama was owned by Colombia at that time. While Theodore Roosevelt tried to negotiate land with the Colombian government to build a canal, he came to believe that they were asking too much. So, he sent a US naval ship to the area. Rebelling Panamanians looked to that as a source of backing, and they succeeded in revolting. They then negotiated quickly with the US. :)
By getting the constitution, it gave the Articles of confederation the power for the national government to enforce laws and defend the US and promote domestic peace because now the national govt. could tax to support an army.