  The formation history of the Small Magellanic cloud (SMC) is unraveled based
on the results of our new chemical evolution models constructed for the SMC,
highlighting the observed anomaly in the age-metallicity relation for star
clusters in the SMC. We first propose that evidence of a major merger is
imprinted in the age-metallicity relation as a dip in [Fe/H]. Our models
predict that the major merger with a mass ratio of 1:1 to 1:4 occurred at ~7.5
Gyr ago, with a good reproduction of the abundance distribution function of
field stars in the SMC. Furthermore, our models predict a relatively large
scatter in [Mg/Fe] for -1.4 < [Fe/H] <-1.1 as a reflection of a looping feature
resulting from the temporally inverse progress of chemical enrichment, which
can be tested against future observational results. Given that the observed
velocity dispersion (~30 km/s) of the SMC is much smaller than that (~160 km/s)
of the Galactic halo, our finding strongly implies that the predicted merger
event happened in a small group environment that was far from the Galaxy and
contained a number of small gas-rich dwarfs comparable to the SMC. This
theoretical view is extensively discussed in the framework that considers a
connection with the formation history of the Large Magellanic cloud.
