Three words: America liberates Ireland!
Two books into this series and my continuing impression is that this Harry Harrison is something like a Harry Turtledove (Lite). This is in no way meant to be an insult or derogatory or bad, as I am finding these books to be just as good as, but different. I feel that Mr. Harrison believes that someone who is interested in speculative fiction / alternate history already has a familiarity with the subject and thus knows the personalities of the historically important characters he places in his novels, thusly, he doesn't feel the need to explain to, expound upon, or otherwise inform the reader why Person X is they way that they are. Whereas, Mr. Turtledove will delight in explaining the personality of said character, often a few times, so that the reader is very aware. Neither is necessarily better than the other, just different.
I also feel that Mr. Harrison is more interested in the "bigger picture" of his story rather than the gritty details, unlike Mr. Turtledove. I'm not saying that Mr. Turtledove doesn't care about his story, but he'll pepper his story with every little gritty detail imaginable. In particular, the Battle for Ireland, whilst the seemingly main plot point of this particular book, was quickly over and done with, whilst if it were done by Mr. Turtledove, I could see it being the entire setting for a couple of hundred pages. As such, these novels, which appear to be roughly 900 to 1,000 pages in total, could easily be three times that if they were written by Mr. Turtledove. Again, neither good nor bad - just different.
With that out of the way, I really enjoyed this second entry in the Stars and Stripes series. I can't say that I'm 100% conversant with the time period to rule upon its plausibility and I still feel that the American Civil War was still resolved far too quickly in the first book - although there are some inclinations in this book that this may not actually be the case. I am very interested to see where the story goes and where it takes us. And I'm very interested to see if there are any ramifications over this increased hostility between the UK and the USA. I think, in our modern day, we often forget the animosity that was present between our two nations before the World Wars of the 20th Century - and even somewhat after. The world FDR envisioned after World War Two did involve breaking the worldwide grasp of European colonial authority, after all.