This paper argues that the perforation of national states by immigration, integration, and trade may signify a critical juncture in the political development of Europe no less consequential for political parties and party systems than the previous junctures that Lipset and Rokkan detect in their classic article. The authors present evidence that party systems are determined in episodic breaks from the past, political parties are programmatically inflexible, and party system change comes in the form of rising parties. They argue that the Eurocrisis and migration crisis have congealed a distinctive structure of conflict in Europe, creating a transnational cleavage centered on the defense of national political, social, and economic ways of life against external actors. This cleavage has much in common with prior conceptions, but the authors adopt a distinctive label to outline its character, sources, and consequences. The emergence of a new cleavage reveals the causal power of social forces in the face of established institutions. The crises have led to the breakthrough of a radical right party in Germany and the rejection of EU membership in the UK, signaling a new era. However, virtually every country contains its own surprises. The focus of the paper is on the general character of conflicts, their relation to the existing structure of party competition, and how they have reshaped party systems. The crises are critical junctures that reveal pressures that have built up over the past two decades. They suggest that party systems are subject to discontinuities rather than incremental change, and that the response of a party system to exogenous change comes from voters rather than parties. The paper explains why cleavage theory can help understand what has happened, emphasizing that political parties are not expressions of already formed, densely organized, and socially closed groups, but rather are constrained by durable constituencies, decentralized decision-making, and self-selected activists. The paper provides evidence that this has indeed happened, showing that the rise of a transnational cleavage is a reaction to reforms that have weakened national sovereignty, promoted international economic exchange, increased immigration, and exacerbated cultural and economic insecurity. The paper also presents evidence that competition on European integration and immigration is structured on the new cleavage, with radical right parties at one extreme and green parties at the other. The paper concludes that cleavage theory explains party system change as a disruptive process rather than an incremental one, with the transnational cleavage having distinct expressions across Europe, reflecting the contrasting effects and differential timing of the economic and migration crisis in different regions. The cleavage arising from transnationalism may endure, as it is grounded in educational opportunities and functional pressures that have given rise to transnationalism.This paper argues that the perforation of national states by immigration, integration, and trade may signify a critical juncture in the political development of Europe no less consequential for political parties and party systems than the previous junctures that Lipset and Rokkan detect in their classic article. The authors present evidence that party systems are determined in episodic breaks from the past, political parties are programmatically inflexible, and party system change comes in the form of rising parties. They argue that the Eurocrisis and migration crisis have congealed a distinctive structure of conflict in Europe, creating a transnational cleavage centered on the defense of national political, social, and economic ways of life against external actors. This cleavage has much in common with prior conceptions, but the authors adopt a distinctive label to outline its character, sources, and consequences. The emergence of a new cleavage reveals the causal power of social forces in the face of established institutions. The crises have led to the breakthrough of a radical right party in Germany and the rejection of EU membership in the UK, signaling a new era. However, virtually every country contains its own surprises. The focus of the paper is on the general character of conflicts, their relation to the existing structure of party competition, and how they have reshaped party systems. The crises are critical junctures that reveal pressures that have built up over the past two decades. They suggest that party systems are subject to discontinuities rather than incremental change, and that the response of a party system to exogenous change comes from voters rather than parties. The paper explains why cleavage theory can help understand what has happened, emphasizing that political parties are not expressions of already formed, densely organized, and socially closed groups, but rather are constrained by durable constituencies, decentralized decision-making, and self-selected activists. The paper provides evidence that this has indeed happened, showing that the rise of a transnational cleavage is a reaction to reforms that have weakened national sovereignty, promoted international economic exchange, increased immigration, and exacerbated cultural and economic insecurity. The paper also presents evidence that competition on European integration and immigration is structured on the new cleavage, with radical right parties at one extreme and green parties at the other. The paper concludes that cleavage theory explains party system change as a disruptive process rather than an incremental one, with the transnational cleavage having distinct expressions across Europe, reflecting the contrasting effects and differential timing of the economic and migration crisis in different regions. The cleavage arising from transnationalism may endure, as it is grounded in educational opportunities and functional pressures that have given rise to transnationalism.