From what you wrote and from my own experience, every country can change its policy when those at its head decide it.
And I do think that there is a certain continuity in the different policies of a country.
Just look at it on the long run and you will see that the foreign policies are rather coherent.
Oh, US policy can be changed certainly. But there are large swaths that never will.
To give you a rather important example of how US policy could change in a rather sweeping way:
Texas decides to pass an amendment to the State Constitution where during presidential elections there will not be a poll of citizens to determine who is awarded the electoral college votes for the state. Instead the votes will be assigned as the State Governor directs.
Now, realistically will this ever happen? To show that it can be argued either way it is completely Constitutional as the Constitution only lays out how to determine how many electoral college votes a state gets and leaves it up to the state to determine how they will be distributed. It would certainly streamline the presidential election process for the state since you can argue that the state is likely to go with the same party as the current Governor and that a Governor who displeases his state by how he assigns the electoral college votes will be voted out of office. But would it be politically possible to pass this (keeping in mind that politicians LIKE being reelected and keeping their office)?
And what about the other way, lets say some group wants to pass a law that does away with the Electoral College system for some reason so they propose an Amendment to the Constitution that will make it a popular vote. After all it is now possible to do such a thing with technology by computerizing the whole system. In addition you could argue that this would make your 'vote' count for the first time ever since you are actually voting for the president rather than simply being polled (as we do now). Of course then there are the other 40 states that realize that if you just go to the 10 most populated states that you have over 50% of the US population, and that with just the top 5 you are doing better than 30%, thereby weakening the political influence of people in states like Wyoming who are relatively important because of the large number of electoral college votes you get compared to the small population. Realistically there is no way this will get pushed through because it steps on the majority of states, even if the end result can be argued to be good.
Effectively trying to change most aspects of the Constitution would be like trying to convince Christians that you are the reincarnation of Jesus, even if that happened to be the case. Their religion specifically says that Jesus will return, but anyone who claims to be him is thought a kook by those who aren't vulnerable to joining cults. Heck, even if you preformed miracles I expect that a large portion of the Christian faith would denounce you as the Anti-Christ who practices Witchcraft (since there is no convenient way to differentiate between them).