A gas turbine engine is provided having an axial centerline, a compressor section, a turbine section, and a combustor section. The turbine section has a turbine first vane assembly that includes an annular first vane inner radial support. The combustor section has an outer casing, an annular combustor, and a unitary inner diffuser structure. The annular combustor has an inner radial flange. The unitary inner diffuser structure includes a compressor discharge, an inner diffuser case, and a tangential onboard injector (TOBI) inseparably attached to one another. The unitary inner diffuser structure further includes an outer radial flange and a TOBI connection flange. The annular FV inner radial support, the combustor inner radial flange, and the TOBI connection flange are secured to one another.