There's a federal law that requires employers to re-employ soldiers at their old duties after their deployment ends.
This soldier got injured while deployed and couldn't resume his normal duties as a state trooper. He asked Texas to put him on other duty instead. When Texas refused, he sued.
Texas argued sovereign immunity, saying it had never agreed to waive its sovereign immunity for this type of claim by an employee.
The Court said, by a 5-4 vote, that such immunity is inherently waived when the common defense of the country is involved.
Honestly, I'm kinda on the dissenters side from a technical legal aspect. This isn't a constitutional claim, and there are plenty of other federal employment laws that don't cover states as employers unless they opt in, either explicitly or implicitly by acts like accepting conditional funding.