Responding to concerns about burden-sharing and aiming to improve internal defence cooperation, act more quickly and harness resource synergies, the European Union (EU) initiated the Permanent Structured Cooperation (PESCO) in 2017. PESCO, however, is controversial. On the one hand, the United States (US) wants greater burden-sharing by European allies whilst concerned about greater European military autarky that would undermine US influence over NATO, Europe/EU and EU member states. On the other hand, at least one European NATO ally wants to leverage PESCO precisely as an instrument to shore up European "strategic autonomy". This tension over competing European defence futures leaves participation by third countries in limbo. Arguably, third-country participation would hinder greater European defence autarky. The article makes the case for the mutual benefits of third-country participation, focusing on Canada. Canada has a major stake in the outcome. NATO is Canada's most important multilateral institution and Europe is Canada's second-most important strategic partner, after the US. Canada's unequivocal strategic interests in Europe have long informed its expeditionary priorities -- from the two world wars, when Canada coming to Europe's defence long before the US proved existential for both parties, to nowadays. Since the 1970s, Canada and Europe have worked consistently together bilaterally beyond NATO to advance regional stability and mutual security interests. Canada's and Europe's defence futures are thus interdependent. Excluding third countries from participating in PESCO would have detrimental consequences for Canadian, European and transatlantic defence interests. In contrast, with third country participation, PESCO will be instrumental to effective transatlantic and transeuropean defence integration. [ABSTRACT FROM AUTHOR]