Would Japanese involvement from the first day materially change a Taiwan conflict?
Short answerYes, but Japan changes the operational picture by its presence and basing even before it becomes an active combatant.
NotesJapan's island facilities and force structure would affect Chinese planning even without active Japanese entry into the war. Island air defenses could support Japanese surface ships operating nearby. Surface-to-air missile sites also provide substantial radar capability. Their presence therefore matters before any decision to engage Chinese forces.