The earliest public release of what came to be the UML was version 0.8 of the Unified Method. It was released for OOPSLA, which took place in October 1995. This was the work of Booch and Rumbaugh, as Jacobson did not join Rational until around that time. In 1996, Rational released versions 0.9 and 0.91, which included Jacobson's work. After the latter version, they changed the name to the UML.
Rational submitted version 1.0 of the UML to the OMG Analysis and Design Task Force in January 1997. Subsequently, Rational combined that version of the UML with other items and submitted a single proposal for the OMG standard in September 1997, for version 1.1 of the UML. This was adopted by the OMG toward the end of 1997. However, in a fit of darkest obfuscation, the OMG called this standard version 1.0. So, now the UML was both OMG version 1.0 and Rational version 1.1, not to be confused with Rational 1.0. In practice, everyone calls that standard version 1.1.
UML 1.1 had a number of minor visible changes from version 1.0.
In adopting UML 1.1, the OMG set up a Revision Task Force (RTF), chaired by Cris Kobryn, to tidy up various loose ends associated with the UML. The OMG released version 1.2 internally in July 1998. This release was internal in that 1.1 remained the official UML standard. You could think of version 1.2 as a beta release. In practice, this distinction hardly mattered, as the only changes in the standard were editorial: fixing typos, grammatical errors, and the like.
A more significant change occurred with version 1.3. Again, though, this is an internal release, so it is not an official OMG standard. However, the changes were more significant this time, most notably affecting use cases and activity diagrams. The amigos' User Guide and Reference Manual were published late in 1998; these books reflected the 1.3 changes before the official 1.3 documents were made public, which caused some confusion.