I see how this works now… play around with the rules until we can establish the US as the “oldest country on earth”. Nice one. 
To be fair, the US has never been conquered or successfully overthrown since the establishment of the Republic.
But Egypt was ruled by Greece and Rome, so I don’t think you can go back further than that.
China came under the rule of the Monguols in 1279, so that might be problematic.
Iceland was ruled by Norway/Denmark for most of its existence.
Actually, under those rules, I think Switzerland might beat the US. The only Swiss revolution I know of occured in the early 1300s. Mind you, it’s quite possible there’s some break in Swiss continuity later than that, and it’s also quite possible that some other nation is older, but I don’t think the US is the winner.
Paging Arnold Winkelried, paging Arnold Winkelried, please.
Turkey? (goes back to Ottoman Empire)
Thailand? (never conquered since its founding in the 13th century)
Under the “single stable government” rule, Switzerland dates from 1848, after the Sonderbund civil war. Swiss history timeline: http://history-switzerland.geschichte-schweiz.ch/
What about Sweden? AFAIK, it’s been under a monarchy without revolutions or invasions since Gustavus Vasa freed it from the Kalmar Confederation. A few assassinations, the adoption of Marshall Bernadotte as his son and heir by the last Vasa king, a bunch of grundlagen creating a parliamentary constitution, but all that within a framework of stable constitutional change, not the sort of abrupt change of government that we have agreed breaks the string.
And I know little about Thai history, but it’s my impression that it’s been an independent monarchy following essentially the same gradual-change process for several hundred years. Dominated as a puppet state, not conquered, by the Japanese in WWII.
(By the way, what do protectorates do to our definitions? If, say, Tonga retains internal autonomy and depends on the U.K. for defense and foreign relations, does that make it un-independent?)
The same would be true of England since 1066 CE.
I apologise for this not being much of a cite, but about ten or fifteen years ago, I heard a little factoid along the lines of there only being nine countries that had managed a century of continual, stable democratic government without revolution, coup, civil war, etc. They included Australia in this, but if you disqualify it, as it wasn’t technically a nation until 1901 (when the nation was peacefully established as a federation of the former colonies), then the answer is actually eight.
This doesn’t specifically address the OP of course, as a country can have major upheavals such as a military coup, and still be the same nation, depending on your definition. However, if the number is eight or nine out of the several hundred nations that exist, then it certainly illustrates how surprisingly difficult it is to keep a place motoring along smoothly for any length of time.
Another thought, how about the Vatican? I think that its continuity can be traced back to the establishment of the Papal States in the Renaissance (though it’s shrunk considerably since then), and “Pope in absolute authority over everything” has been the rule for all of that time.
Actually, I was surprised. Until I started looking, I assumed the US was a fairly young country as these things go.
That said, any government is going to evolve as time goes on. It’s debatable whether you can really say that the United States that existed in 1789 is the same country that exists in 2005 or existed in 1776. Is the Fifth Republic of France a different country from the Fourth Republic or the Second Empire or the Directorate? Was England the same country as the United Kingdom? Is Germany a different country since it reunited?
But if you ignore governments and only count people, then Tanzania is the oldest country on Earth - that’s apparently the first place people lived and some of their descendants have lived there ever since.
There was a break in continuity of residing at the Vatican in Rome with the Popes of Avignon in the 14th century.
Obviously the US isn’t the oldest country on earth, but we may have the longest continuous present government. One could make the argument that the extensions of the franchise and direct election of Senators make this a different government than the original, though I’d disagree.
Where did you get that idea? It’s had civil war and William of Orange. The latter William may have had the support of Parliament, but he certainly backed it up with a vast fleet and a large body of troops.
Well, again, it’s that definition problem, but I definitely wouldn’t include Turkey which is a thoroughly twentieth century creation. The Ottoman state was dominated by Turks, but was defintely multi-ethnic with boundaries far beyond those Turkey currently has.
Thailand was briefly conquered or at least shattered by Burma in the 1760’s/1770’s - the capital was destroyed, the west overrun and state structure collapsed ( various provincial warlords declaring independence ). The dynastic kingdom of Ayyuthia, in place since the 14th century, came to an end in 1767. This produced an interregnum of around a decade before a combination of Chinese pressure on Burma’s northern border ( Burma ultimately fought off a series of four Manchu invasions ) and the rise of a particularly capable Thai leader rose to push them back and restablish Siam under a new( the current ) dynasty, which however was overthrown after a few years and replaced by the current Chakri dynasty in 1782. Governmentally then, it is just slightly younger than the United States.
Oh and at its height in the late 19th century Siam was a much larger state ( or at least hegemony ), including all of Laos and Cambodia and wider areas in the Malay penninsula as either vassals or outright demesnes. France extracted the eastern territories in Indochina by force in the very late 19th/early 20th centuries. The Malay possessions were bargained away to the British.
- Tamerlane
Sorry - editing on the fly is always my downfall :). Of course that first ( very short-lived, 1776-1782 ) dynasty isn’t the current one.
- Tamerlane
Wouldn’t Cromwell’s ascent to power count as the government being successfully overthrown? I say that if you have the clout to militarily defeat and subsequently behead the king, you have waged a successful revolution and broken the chain of peaceful transitions of government.
My cite for Cromwell and his Revolution. There is tons of info online, this is simply the webpage I found first.
Hmm. Duration of continuous government under present constitutional structure? That’s kind of narrow, isn’t it? Canada would only date to about 1982 by that definition.
A cultural definition would make more sense, I think. China would count as the oldest continuous culture, as other posters have stated. Egypt may be older, but I don’t believe the present language/culture of what we call ‘Egypt’ is continuous with that of the Pharaohs. And of course Sumer was overrun even earlier than Egypt.
I’m wondering whether Incan, Quechuan, Aztec, or Mayan cultures are old enough and continuous enough to work…
So? That’s like saying the US only dates back to 1865. Where two branches of government or two parts of a country disagree and the issue is resolved by the use or threat of force, I think it’s fair enough to say that the country remains the same if there are no material changes to the habits and arrangements of governance. The US after the civil war had the same constitution and so on as before. England continued after slotting in a new monarch in the place of an old one - the facts on the ground didn’t change much.
I think Cromwell counts as an interruption of governance since he disrupted BOTH branches of authority (Monarchy and Parliament), but it’s sort of tricky that afterwards things picked up where they left off - a hiatus more than a break, if you will.
Similarly, where a country decides to change its form of government and does so peacefully and in accord with the legal provisions of its former government, it seems a bit odd to say that it has started afresh.
If de Gaulle drafts a new constitution, 79% of French citizens vote for it in a referendum, and it is adopted, then surely the Republic of France has been operating according to its own laws and constitution throughout the whole process, and has been in continous existence?
Sunspace - Culturally, I don’t believe the Amerindian civilizations you refer to exist anymore than the Vikings, Celts or Romans do. Having people of related ethnicity with some inherited traditions living in the same geographic area does not mean they are part of the same culture.
Incidentally, by China are we referring to the Han only? I always get lost in the maze of dynasties, regions and languages, even though its no more complicated than European history - probably because I came to it late in life 
Denmark claims to be the oldest kingdom on earth
That might well be a valid point of view. The point here is that there’s no good answer, because it all comes down to opinion and rules. You can set up the rues however you like.