"..this imaginary community of a population identified as a nation, became the extreme subject of political action as guarantor of full legitimacy of the State , as repository of popular sovereignty" (from Ideas de naciòn, Enrique Moradiellos)
"illegal" is used here in the title, as a technical euphemism to stress over the misthought and manipulation by nationalistic movements fostered by increasingly pervasive populism over the term and subsequently the role of the nation as such.
The proposed large neon letering "all nations are illegal" is a provocative statement reflecting over the very much debated concept of "nation", constantly activated as contemporary political propaganda lever to instrumentally evoke dramatic counter effects on society of globalisation processes mainly accused of causing distorted wealth distribution and local labour substitution, due to uncontrolled immigration flows.
Starting from the false assumption that nation itself is natural and comes prior to the State, the project is questioning therefore the legality of national sovereignty as such, and the everywhere growing use of nationalism as a political mechanism to generate consensus to foster boundaries  and enable erection of both real and artificial borders as safeguard from unwanted access.
Therefore, from the (H)Hegelian definition of Nation as People's Spirit to the hegemonic strategy of the world as Global Village, it is clear that the concept of Nation has no justification whatsoever.