What do the authors say at the outset is the "supreme lesson" of the war of 1812?

The authors say at the outset is the “supreme lesson” of the War of 1812, The leading a divided and apathetic people into war is a bad idea and for two years, the Americans and British fought to a standstill in Canada. Perry and then on Lake Champlain, a British force invaded the Chesapeake region and burned the new capital of Washington to the ground in August 1814. They then were beaten off in the battle at Fort McHenrynear Baltimore that inspired Francis Scott Key to write the Star Spangled Banner. Finally, the victory of Gen. Andrew Jackson, over the British at New Orleans in January 1815 gave a boost to national morale, even though, because of slow communications, it was actually fought after the Treaty of Ghent had been signed, officially ending the war.