Isolationism would not protect the US from terrorist attacks because modern terrorism is transnational, decentralized, and borderless. Retreating from global affairs would not eliminate the threat due to the following critical vulnerabilities: Many modern attacks are carried out by domestic extremists or radicalized individuals already inside the country. These actors often do not require direct foreign direction or infiltration; they are inspired by ideologies disseminated globally via the internet.
Terrorist networks and state sponsors can project power, disrupt critical infrastructure, and incite violence globally without ever crossing a physical border. Abandoning international alliances and intelligence sharing would forfeit the ability to disrupt terror plots at their source. Failing to monitor and address instability abroad allows safe havens for groups to organize, train, and fund operations targeted at the US.
A purely isolationist stance also disrupts vital partnerships and intelligence-sharing networks with international allies, which are the primary defense for identifying and intercepting transnational threats before they materialize on US soil.