The challenge in a globalized economy is that what may be a monopoly in the US is likely competing against several global corporations within a given industry. Leveraging anti-trust regulations against that dominant US player may limit its ability to compete with global rivals, leading to the demise of that company and the dominance of foreign corporations in the industry.<p>In the past, when the US economy was more isolated and the level of global competition was much lower than it is now, breaking up monopoly corps in the US wouldn't leave the resultant child corps vulnerable to foreign competition.