In one word, relieved.

Quote from volume 3 of his series The Second World War:

"...No doubt it would take a long time. I expected terrible forfeits in the East; but all this would be merely a passing phase. United we could subdue everybody else in the world. Many disasters, immeasurable cost and tribulation lay ahead, but there was no more doubt about the end.

Silly people, and there were many, not only in enemy countries, might discount the force of the United States. Some said they were soft, others that they would never be united. They would fool around at a distance. They would never come to grips. They would never stand blood-letting. Their democracy and system of recurrent elections would paralyse their war effort. They would be just a vague blur on the horizon to friend or foe. Now we should see the weakness of this numerous but remote, wealthy, and talkative people. But I had studied the American Civil War, fought out to the last desperate inch. American blood flowed in my veins. I thought of a remark which Edward Grey had made to me more than thirty years before--that the United States is like a "gigantic boiler. Once the fire is lighted under it there is no limit to the power it can generate". Being saturated and satiated with emotion and sensation, I went to bed and slept the sleep of the saved and the thankful."