The author states that “the war itself was mostly a political and military disaster for the country.” Why is this the case?

Not all congress was on board in joining the war, and it caused  a massive damage to US military

Explanation:

In the war of 1812, United States battled against the British Empire After the British empire continually use its influence to take Over American Ships during their trading activities.

Congress was divided on how to responded to British' actions.

Some of them believe that they're too strong for United States to handle so USA should avoid war at all cost. But some of them believe that the take overs will hurt US economy in the long run so they have no choice beside war.

The war eventually ended in statement, but the British empire agreed to not take over American ships anymore. But, the war itself cost a lot of money from government budget and massive casualties for US military.

managing the colonial war effort while also forcing for and moving towards america's independence was the major agenda of the debate in course at the second continental congress. the delegates at the congress also adopted the united states declaration of independence, in so doing acting as the ''de facto'' national government of what later became the united states of america