Federal antitrust officials have fired another shot in their war on monopolists, and this time it will likely remind executives at even the smallest of companies that their modest deals are not exempt from scrutiny.

The Department of Justice filed suit Friday to block mainframe computer software maker Compuware Corp.’s $168 million acquisition of a competitor — a surprise turn of events to a community of antitrust watchers focused on far bigger alleged lawbreakers, like Microsoft Corp.