The phrase "union with Austria" refers to several distinct historical events and proposals:
The most infamous: Hitler's forced annexation of Austria into the Third Reich (March 1938), approved by a manipulated plebiscite. Violated Treaty of Versailles.
Wartime discussions of a closer political/economic union between the two Central Powers, never realized due to defeat.
1950s Stalin-era idea: a unified, neutral Germany including Austria as a buffer. Rejected by West; Austria chose neutrality via 1955 State Treaty.
Created the Dual Monarchy: union of Kingdom of Hungary and Kingdom of Croatia-Slavonia under the Habsburg crown (not "with Austria" but of Austrian lands).
Archduke Franz Ferdinand's unrealized federalist plan to restructure the empire into semi-autonomous states.
Context is essential: "Anschluss" specifically denotes the 1938 Nazi annexation; other uses refer to constitutional or geopolitical proposals.