When the United States and some of its allies in 2003 invaded and destroyed Iraq on false pretenses — and without Iraq having ever invaded (much less destroyed) any of the invading countries — this was actually within the scope of the invaders being liberal countries, because a nation’s sovereignty isn’t at all respected in traditional liberal thought. This also is the reason why some of the same nations invaded and destroyed Libya in 2011, and Syria since 2012. Neither of those two invaded countries had ever invaded — much less destroyed — any of their invaders; but, in all of these cases, such invasions were accepted by the populace within each of the invading countries, all of which invading countries considered themselves to be liberal nations. Why do liberals (and not only conservatives) so routinely accept barbaric aggressions by their own country? Here is the reason (and it needs to be read slowly and carefully, in order to become understood, because what follows is densely packed with meaning; the subject here is sufficiently deep to reach the core of things, like drilling through hard rock — it’s necessarily slow going):
A nation’s sovereignty means that the residents in a land possess the ultimate authority over that land, regardless of what its ‘owner’ might happen to be: a foreign king, an international corporation, or even a domestic person who is one of the people who live there. Consequently, whereas an authentic revolution by the residents within a country, to overthrow and replace their government — or else a vote to secede — is acceptable in the concept of national sovereignty (and is recognized as “the right of self-determination”), no foreign invasion is (and this includes any internal invasion to defeat a secession), unless the invasion is authentically a response to a real and present danger of, or else in direct response to, an invasion by the country (or region) that's being invaded. This is the concept of national sovereignty: the residents rule — no foreigner does. However, the concept of national sovereignty is fundamentally alien to liberals.
Liberalism is instead dominated by the concept of the individual’s right to property, which is the fundamental right in liberalism, upon which all other rights are (in traditional liberalism) based.
As one summary of John Locke’s political theory put this most clearly:
“The theory of property was understood to be central to the structure of Locke’s argument in the Second Treatise in that it serves as an explanation for the existence of government and a criterion for evaluating the performance of government. Locke’s individualist, private property stance was not always admired or believed to be without flaw, but criticism was leveled within the context of Locke’s claim to a place as a liberal philosopher."
However, Adam Smith, writing in Locke’s tradition 87 years later, provided a clearer case than anyone up till his time, for the right to property being the fundamental right, the right which governments are instituted specifically in order to advance and to protect; and, so, here that basic statement is, in The Wealth of Nations, Book V, Ch. 1, Part 2:
… Wherever there is a great property, there is great inequality.
For one very rich man, there must be at least five hundred poor, and the affluence of the few supposes the indigence of the many. The affluence of the rich excites the indignation of the poor, who are often both driven by want, and prompted by envy to invade his possessions. It is only under the shelter of the civil magistrate, that the owner of that valuable property, which is acquired by the labour of many years, or perhaps of many successive generations, can sleep a single night in security. He is at all times surrounded by unknown enemies, whom, though he never provoked, he can never appease, and from whose injustice he can be protected only by the powerful arm of the civil magistrate, continually held up to chastise it. The acquisition of valuable and extensive property, therefore, necessarily requires the establishment of civil government. …
The causes or circumstances which naturally introduce subordination, or which naturally and antecedent to any civil institution, give some men some superiority over the greater part of their brethren, seem to be four in number.
The first of those causes or circumstances, is the superiority of personal qualifications, of strength, beauty, and agility of body; of wisdom and virtue; of prudence, justice, fortitude, and moderation of mind. …
The second of those causes or circumstances, is the superiority of age. …
The third of those causes or circumstances, is the superiority of fortune, the authority of riches …
The fourth of those causes or circumstances, is the superiority of birth. Superiority of birth supposes an ancient superiority of fortune in the family of the person who claims it. …
In liberalism, a person’s “superiority” or “inferiority” is measured by the amount of wealth he or she “owns.” (Otherwise called “net worth.”) This is the professional economist’s belief in the inevitable rightness of “the free market”: an economist, by his/her being a professional who is devoted to that theory, is committed to this type of hierarchy or inequality — the belief that the more property one owns, the better that person is; and, so, the less that one owns, the worse he or she is.
Precisely how this system contrasts in any fundamental way with conservatism is not clear (and liberals especially don’t discuss it), but Smith’s central case was actually against mercantilism, which, in recent times, is part of nationalism — mercantilism is the argument for any nation to apply tariffs and other protectionist measures in order to block foreigners from “grabbing” business away from the residents (the subjects to the local sovereign) within the given nation. Adam Smith’s argument was against sovereignty, not in support of it. Property-rights and property-obligations — obedience to, and governmental protections of, these rights and obligations — are at the very foundation of liberalism, whatever one might happen to consider either “liberalism” or “conservatism” to mean.
Progressivism means something totally different than either liberalism or conservatism: it is the belief not in “natural law” nor in any “God’s Law,” but instead in natural worth: Worth inheres in any sentient being, because it is sentient and can therefore experience joy (positive) and/or misery (negative). No sentient being can be property — it can only be an owner. It can be conquered, but it can’t be owned. It can be a dependent, but it isn’t itself owned, not by anybody but itself. By extension, this applies to the residents on any particular land; and, since they and they alone own themselves, they collectively are the sovereigns over that land; they alone possess the natural right to rule there. In the view of a progressive person, consciousness (especially the polarity between joy and misery) is the actual basis of worth; property isn’t. Materialism is no longer the basis in the realm of values, but “spirit” (consciousness) is that. The goal is societal well-being, not merely personal wealth. In a progressive light, invasions are evil (because negative, misery-inducing). But, in a liberal light, they aren’t evil, and they can even turn out to be good, if the result is ‘success’: “victory.” (Consequently, empires are acceptable to liberals.)
Consequently (for recent examples), no progressive supported nor endorsed the invasion of Iraq in 2003, nor the invasion of Libya in 2011, nor the invasion of Syria since 2012. (And, to call that invasion of Syria, by tens of thousands of foreign jihadists who were paid by Saudi Arabia and armed by the United States, a ‘civil war’, as is commonly done, is simply a lie, just as bad as the lie that Saddam Hussein was hiding WMD in 2002.)
This is the concept of national sovereignty. It isn’t an economic concept; it isn’t a philosophical concept, it is instead a statement about sentience and what sentience entails; it is a scientific concept, and it is the very foundation of any progressive (i.e., science-based) political theory.
This concept, national sovereignty and all the rest of progressivism, does not preclude some type of world government gradually emerging, so long as that occurs 100% by means of democratic processes, and respects the national sovereignty of each and every one of the participating nations, and so long as all nations are honestly welcomed to join, on the same basis as the existing member-nations did. Only in this way, and by democratic process from the bottom to the top, would it even be possible for a world government to develop as being a force for peace in the world, instead of as a force for some type of international dictatorship (and thus as a force for even more war).
So: whereas liberals don’t respect a nation’s sovereignty, progressives do — it’s a basic part of progressivism.