Old War, New Generals
The first major OS war was between VMS and Unix. VMS fans berated the absence of certain “features” in Unix, such as record-level locking, while Unix enthusiasts considered VMS an over-wrought monstrosity. (I’ve seen the manual set for VMS, and the idea is not unfounded.)
Over time, VMS became relegated to legacy systems and high-end corporate clusters, and AT&T sold off the Unix rights. The NeXT system, developed by erstwhile Apple CEO Steve Jobs, tried to get Unix into the offices and dorm rooms of individuals.
As the market space for VMS became saturated, developers left DEC for Microsoft, providing strong influence for the Windows NT operating system. (See the 100ns timestamp resolution and the alternate file stream operations? Those came from VMS.)
When Steve Jobs returned to Apple, and brought NeXT with him, it was only a matter of time until the Macintosh ran Unix. Mac OS X shows the profound influence of Unix in its low-level command structure, and the influence of NeXT in the user interface. (See the object system with class names starting with NS- ? That’s NextStep in there.)
When Windows XP brought the NT kernel to the desktop, and Macintosh brought Unix to the desktop, it became just another stage in an old competition: VMS vs. Unix.