  We study systematically the production of strangeness in nuclear reactions
from SIS to SPS energies within the covariant hadronic transport approach HSD.
Whereas the proton and pion rapidity distributions as well as pion transverse
momentum spectra are well described in the hadronic transport model from 2-200
AGeV, the $K^+$ and $K^-$ spectra are noticeably underestimated at AGS energies
while the $K^+$ spectra match well at SIS and SPS energies with the
experimental data. We conclude that the failure of the hadronic model at AGS
energies points towards a nonhadronic phase during the collision of heavy
systems around 10 AGeV.
