Thomas Jefferson never asked Monroe to be Envoy Extraordinary to France; instead, he merely sent his protégé a note informing him that Monroe had already been nominated to the position. He was to assist Robert Livingston in his quest to purchase a significant port on the Mississippi River, which was at the time largely controlled by the French government. Jefferson had proceeded with the appointment because the right to use New Orleans as a port of deposit had been revoked, causing outrage throughout the American West. Jefferson needed the problem to go away, and so he turned to Monroe in January 1803, asking for "a temporary sacrifice to prevent the greatest of all evils in the present prosperous tide of our affairs." Jefferson knew that Monroe could never turn down such a request. By January 11, the Senate confirmed Monroe and he was on his way. To pay his way, Monroe sold his plates and china to Madison.

This second journey to France was markedly different than his first, for he departed with the full backing and good graces of the administration–an administration he deeply admired. The trip became increasingly complicated, as Monroe was also prepared to work with England or Spain to gain the access the U.S. required, alliances which France found troublesome and likely helped push his case in Paris. Nevertheless, after weeks of legislative wrangling, Monroe departed on March 2 from New York with authorization to spend fifty million francs, just a little over nine million dollars, purchasing as much of New Orleans and Florida as France was willing to sell.

Much of the work had been completed before Monroe even arrived in Europe–in fact, far more work than had originally been proposed. On April 11, the French Foreign Minister had asked Livingston whether the U.S. would be interested in purchasing the all of Louisiana. Livingston had protested that America only wanted New Orleans, but the French pointed out that without a port, the rest of the territory was useless to France. Livingston offered twenty million livres, but Minister Talleyrand demurred. Later, it turned out that Napoleon himself had ordered the sale. Finally, the French government and the two American ministers settled on a price of sixty million francs, plus another twenty million to settle claims against the land. At a dinner party on May 1, the final terms were agreed upon. Monroe and Livingston agreed to the sale, even though the cost exceeded their charge–believing rightly so that Jefferson would still jump at the opportunity to double the size of the United States. Despite the fact that no one really knew how large Louisiana was–for instance, no one was sure whether it included Western Florida–Monroe and Livingston recognized it was huge.

The news was greeted with amazing excitement in America–the threat of foreign incursions had been removed and America had endless tracts of land now to expand. The new land allowed for an "empire of liberty." Jefferson was thrilled. The act also catapulted Monroe was some prominence to the highest levels of the nation's leaders–many now expected it would only be a matter of time before the former governor of Virginia became president.

Not surprisingly, Monroe's "temporary" assignment overseas soon turned into a more permanent one. From 1803 to 1807, he served as minister to Britain–perhaps the highest post in the diplomatic corps. His long- standing animosity towards the crown was tempered by the warm reception he found there. He continued the work of his predecessors, hammering out treaties involving commercial alliances and entertaining with the British court. In 1806, he successfully negotiated a new treaty that relaxed trade restrictions between the two countries–but since it included no provisions dealing with the impressment of seamen by the British, Jefferson refused to ask for the treaty's ratification. Monroe also spent several aggravating months in 1805 trying to convince Spain that the Louisiana Purchase included West Florida and the surrounding lands–all to no avail.

When his tour in Britain ended, he returned to Virginia and still smarting from the rejection of his treaty (Monroe felt that he had gotten the best deal possible at the time), he ran a protest campaign against his friend Madison for the presidency in 1808–blaming Madison for the treaty's rejection. Monroe won little support and Madison went on to the President's House.

On bad terms now with Jefferson and James Madison, Monroe served briefly in the Virginia assembly from 1810–1811 and then was reelected governor. Once more, though, his country's need would interrupt his life. He only served three months as governor for Madison asked him to accept the post of secretary of state. Madison needed his old friend to help him govern among a Federalist party growing in strength and factions in their own Republican party. The appointment reunited the powerful triumvirate of Monroe, Madison, and Jefferson.

His age and experience (including being perceived as a national hero for the Louisiana Purchase) helped Monroe reassert his power in Washington. With growing hostilities between Britain and the United States, and Monroe's experience as a British diplomat, Monroe played a key role in the run-up to the War of 1812. He won the backing of the "War Hawks" for administration policies and, when war broke out, asked for a post in the army. Sadly, despite his great efforts to win a long-coveted command position in the military, Monroe was blocked by Secretary of War John Armstrong, who felt that Monroe had usurped the place in history as negotiator of the Purchase that rightly belonged to Armstrong's nephew Robert Livingston.

Monroe got revenge in 1814, when the British invaded and burned Washington–an incident that was blamed on inadequate preparations by Armstrong (Monroe had supervised the evacuation of government offices and fled only as the British arrived in the city proper). Madison fired the secretary of war and appointed Monroe to that position; now he held the two most powerful cabinet posts simultaneously. Monroe got a chance to orchestrate the defense of Baltimore and succeeded admirably, sleeping for days at a time on a cot in his office. With the British vanquished, and never being one to pass up a chance to reform and strengthen government, Monroe moved ahead with reforms in the War Department and even came up with early alternatives to the draft. The matter was later dropped before implementation.

As Madison's term as president wound down, there was little doubt where the next president would come from. Monroe, a former ambassador to Spain, France and Britain, secretary of war and secretary of state, former senator, and two-time governor of Virginia, seemed a natural choice.