In a war with Russia? I mean if war broke out today.
I voted
other.
In a global, non-nuclear conflict: the U.S.A wins.
In a regionally limited, conventional conflict, one fought in Russia's 'backyard': Russia
could win.
In a mutual, limited exchange of nuclear weapons: the U.S.A. wins with heavy loss of infrastructure.
In a global, nuclear war: both Russian and the U.S.A. face mutually assured destruction.
Any conventional global world war reminiscent of the two world wars of the last century where alliances of nations combine their military-industrial might would be fought over much greater geographical distances than ever before witnessed in human history. Such a conflict would be won and lost both at/from sea and in the air. We've surpassed militarily the concept of
fire and forget to the level where we (the U.S.A.) can kill enemy targets over half the globe away.
While sub-superpower nations such as China and Russia wield similar extreme range capabilities, they would first have to mobilize massively their conventional ground and sea forces for deployment to distant--even near distant--theaters of operation. The U.S.A. on the other had, has the key advantage of some of its most elite fighting forces already dispersed, trained, supplied and ready to fight in place around the planet .
Additionally, the U.S.A.--in conjunction with the most powerful naval force in human history--also maintains several Marine Expeditionary Units with fleets in key strategic would be theaters around the world. MEUs are completely autonomous Marine Corps units capable of fighting sea, land and air battles, for limited duration, and can respond to crisis hotspots within hours or days.
At length, while China, Russia and whatever other nations join them in mustering and preparing for deployment, the U.S.A. has the capability to already be there, ready to fight short term, with overwhelming force while corps, divisions and fleets back home pack up at railhead and ship to ports for deployment.
The other side of the Russia/China on U.S./N.A.T.O. global conflict is
technological warfare. Tactics such as mass DDOS, electronic warfare, infrastructure hacking, unconventional sleeper units bearing man-portable nukes and EMP. Lastly, any offensive first move by Russia/China could be spearheaded by a series of well coordinated terror attacks on U.S. and European soil--including nuclear, biological or chemical weapons.