The Constitution gives the authority to declare war to Congress, but often a hawkish President seems to be the authority that pushes for war - sometimes against the wishes of the House and Senate.
Has there ever been an instance where a reluctant President was forced into war by a hawk Congress? It would be interesting to know if a President was ever forced to start a war that he did not back.
On mobile on the train, so my answer isn't going to be terribly well written, though I hope others will expand upon it properly.
I'm afraid there's not much of an interesting answer here. The United States has only officially declared war 11 times in 5 wars (many are grouped together in the World Wars).
The closest thing to what you seem to be interested about would be the declaration of war against Spain, as McKinley was not a man who wanted to lead the United States into war, particularly a war that the United States starts themselves.
As you know, the United States was pretty anti-imperialism, especially in the Americas. As Cuba was fighting for independence, there was a wish from certain sectors of the United States to support the Cubans in these times. But most importantly a diplomatic solution was wanted, especially by President McKinley, so as to not damage any sort of trade.
The US and Spain negotiated a peace over Cuba, which was granted autonomy within the Spanish Empire, much like the US's neighbours to the north, Canada.
It seemed like McKinley had achieved his aims. Until the USS Maine was destroyed off the coast of Cuba. Now McKinley had shown his want for peace and urged Congress not to be rash about the destruction of the Maine. But as a pragmatic man, he asked for an increase to the defence budget, which he received.
Eventually it was concluded (by the Americans) that the USS Maine had been destroyed by an enemy attack. However Spain did not want a war and said they authorised no such attack, the explosion must have been a ship malfunction or sabotage, but that's another story.
Given the American conclusion that it was a Spanish attack, pressure for war mounted, and for a while McKinley kept pursuing diplomatic solutions to the crisis, to the point where he was nearly standing against much of the will of Congress and the people.
Eventually, he asked Congress to decide on the matter. He didn't ask for a declaration of war, because he knew what he was doing was tantamount to it anyway, and he didn't want to be the one who asked for war.
Even though McKinley wanted to avoid war at all cost, he knew that once diplomatic channels had been closed he had no choice.