My understanding of this book is that Bertrand Russell thought nationalism (more commonly called "patriotism" and seen as a good thing) was one of the main causes of war by creating the feeling that one's country is always right, which caused the suspicion against other countries and peoples, and that the "possessive" feeling for material goods that leads to greed and the desire for power that not only makes the leader of a country want to take territory from another in order to increase his power, but was the cause of colonialism, and in his view is a part of the competition that is inherent in capitalism, although that view of capitalism can be disputed.
I think he viewed free trade as an important step, although a minor one, in reducing tensions among countries by showing that other countries had value to offer to us, which would make us more likely to want to cooperate with them in international affairs and therefore would reduce the likelihood of war and would show that other people in the world are more similar to use than they are different. It seems he also realized that the labor movement would be one of the principle opponents to free trade, although he didn't explicitly say that. Also I don't think he could have envisioned how rapid globalization caused by free trade would become and that it would not just be displaced workers in opposition to this change.
I feel it is refreshing reading a book about pacificism that was written in 1917, when World War I was still raging, and by someone who was thrown in jail for writing his views against senseless war and his ideas how it can be stopped, even if his idea of a one world government and the abolition of capitalism can be called unrealistic. Also keep in mind that the League of Nations wasn't created until 1919 and the UN in 1945.
This book is definitely worth reading, but look for it at a library because such a small book isn't worth an exorbitant price.
Page 84: "Universal free trade would indubitably be of economic benefit to mankind, and would be adopted tomorrow if it were not for the hatred and suspicion which nations feel toward another."
Page 36: "At present, owing to the fact that all industrial changes tend to cause hardship to some section of wage earners, there is a tendency to technical conservatism on the part of labor, a dislike of innovations, new processes, and new methods."
Page 85: "National pride as it exists now, is almost exclusively concerned with power and dominion, and with its capacity of enforcing its will against the opposition of other nations."
Page 88: "Men must learn to be conscious of the common interests of mankind in which all are at one, rather than of those supposed interests in which the nations are divided."