"he had declined to discuss whether the US would defend it" [declined to discuss]
Trump's explicit refusal to affirm the US defense commitment — in the context of a bilateral summit covering trade and diplomacy — directly evidences the conversion of security guarantees into negotiating ambiguity.
Reasoning from this article
The article documents Trump simultaneously warning Taiwan against independence, refusing to confirm US defense commitments, and discussing arms sales 'in great detail' with Xi — all within a summit framed around broader US-China relations including trade. This pattern generalizes: when a patron power faces economic interdependence with an adversary, security guarantees to third parties become fungible assets in broader negotiations rather than fixed commitments. The structural implication is that alliance credibility degrades as a function of patron-adversary economic entanglement.