Understanding nationalism from a left perspective requires that we understand what a nation is. In Marxist theory, a nation is a "historically constituted, stable community of people, formed on the basis of a common language, territory, economic life, and psychological make-up manifested in a common culture." Nationalist movements are therefore struggles for sovereignty. Nationalism must be understood from its relation as a nation. Nationalism of oppressor nations and nationalism of oppressed nations are both struggles for sovereignty, the core difference being that oppressor nations have already gained the ability to determine their own future, and this progressive movement turns into its opposite, supremacy.

This is why we can say that the nationalist movements in Palestine, Ireland, indigenous Americans, and so forth are undoubtedly progressive in gaining that right to self-determination. This is also why we can say that Euro-American nationalism is a supremacist ideology. This is why nationalism can be difficult to understand in relation to internationalism, but the simple truth is that an internationalist stance requires the right of self-determination for all nations. This is the bedrock of internationalism and a greater international human identity. It isn't an erasure of culture and national identity, but a celebration of all of them.

TL;DR: nationalist movements in oppressed nations like Palestine good, nationalist movements in oppressor nations like the US bad.