Roosevelt argued that attempts to isolate the United States from the rest of the world are misguided, dangerous, and unsustainable. He framed isolationism as shortsighted in the face of global threats and stressed that American security, democracy, and freedoms depend on actively engaging with the world and helping to organize collective security. Key points Roosevelt highlighted across his Four Freedoms era rhetoric and related addressings:

• Interdependence of nations: The survival of democracy and freedom in the world was tied to a stable international order, not to a walling off from global events.
• Consequences of isolation: Attempts to retreat into isolation would leave the United States more vulnerable to aggression and would undermine the very principles—liberty, equality, opportunity—that define the nation.
• Responsibility to counter aggression: Facing the spread of totalitarianism and the threats to free peoples abroad required broader international cooperation, support for allies, and readiness to defend democratic norms.
• Liberal internationalism as a protection of American ideals: Engaging abroad was presented as a means to safeguard the freedoms at home, rather than a departure from American principles.

If you’d like, I can pull specific quotations from Roosevelt’s Four Freedoms speech and related statements, or summarize his keynote proposals (such as aiding allies, increasing production and military readiness, and shaping a postwar international order) with precise references.