Critics of nationalism have argued that it is often unclear what constitutes a "nation", or whether a nation is a legitimate unit of political rule. Nationalists hold that the boundaries of a nation and a state should coincide with one another, thus nationalism tends to oppose multiculturalism. In doing so, nationalism serves to marginalize minorities who live within a nation-state but do not share the necessary characteristics to be considered part of the nation. It can also lead to conflict when more than one national group finds itself claiming rights to a particular territory or seeking to take control of the state.
Philosopher A.C. Grayling describes nations as artificial constructs, "their boundaries drawn in the blood of past wars". He argues that "there is no country on earth which is not home to more than one different but usually coexisting culture. Cultural heritage is not the same thing as national identity".
Nationalism is inherently divisive because it highlights perceived differences between people, emphasizing an individual's identification with their own nation. The idea is also potentially oppressive because it submerges individual identity within a national whole, and gives elites or political leaders potential opportunities to manipulate or control the masses. Much of the early opposition to nationalism was related to its geopolitical ideal of a separate state for every nation. The classic nationalist movements of the 19th century rejected the very existence of the multi-ethnic empires in Europe. Even in that early stage, however, there was an ideological critique of nationalism. That has developed into several forms of Internationalism and anti-nationalism in the western world. The Islamic revival of the 20th century also produced an Islamist critique of the nation-state. (see Pan-Islamism)
At the end of the 19th century, Marxists and other socialists and communists (such as Rosa Luxemburg) produced political analysis that were critical of the nationalist movements then active in central and eastern Europe, although a variety of other contemporary socialists and communists, from Vladimir Lenin (a communist) to Józef Piłsudski (a socialist), were more sympathetic to national self-determination.
In his classic essay on the topic George Orwell distinguishes nationalism from patriotism, which he defines as devotion to a particular place. Nationalism, more abstractly, is "power-hunger tempered by self-deception."
For Orwell, the nationalist is more likely than not dominated by irrational negative impulses:
There are, for example, Trotskyists who have become simply enemies of the U.S.S.R. without developing a corresponding loyalty to any other unit. When one grasps the implications of this, the nature of what I mean by nationalism becomes a good deal clearer. A nationalist is one who thinks solely, or mainly, in terms of competitive prestige. He may be a positive or a negative nationalist—that is, he may use his mental energy either in boosting or in denigrating—but at any rate his thoughts always turn on victories, defeats, triumphs and humiliations. He sees history, especially contemporary history, as the endless rise and decline of great power units and every event that happens seems to him a demonstration that his own side is on the upgrade and some hated rival is on the downgrade. But finally, it is important not to confuse nationalism with mere worship of success. The nationalist does not go on the principle of simply ganging up with the strongest side. On the contrary, having picked his side, he persuades himself that it is the strongest and is able to stick to his belief even when the facts are overwhelmingly against him.
In the liberal political tradition there was mostly a negative attitude toward nationalism as a dangerous force and a cause of conflict and war between nation-states. The historian Lord Acton put the case against "nationalism as insanity" in 1862. He argued that nationalism suppresses minorities, it places country above moral principles and especially it creates a dangerous individual attachment to the state. However Acton opposed democracy and was trying to defend the pope from Italian nationalism. Since the late 20th century liberals have been increasingly divided, with some philosophers such as Michael Walzer, Isaiah Berlin, Charles Taylor and David Miller emphasizing that a liberal society needed to be based in a stable nation state.
The pacifist critique of nationalism also concentrates on the violence of nationalist movements, the associated militarism, and on conflicts between nations inspired by jingoism or chauvinism. National symbols and patriotic assertiveness are in some countries discredited by their historical link with past wars, especially in Germany. British pacifist Bertrand Russell criticizes nationalism for diminishing the individual's capacity to judge his or her fatherland's foreign policy.  Albert Einstein stated that "Nationalism is an infantile disease. It is the measles of mankind".