The question is: who can stop it?
Certainly not "the people," who never approve of these ruinous wars that we've had after WWII (the last we won). They are too unorganized at present anyway -- utterly fragmented -- and so have little leverage to wield.
That leaves the elite sectors of society, who act collectively in their sector's material interests. We must then identify which ones stand the least to gain from WWIII, and therefore the most to lose, since the downside is infinite for all sectors (nuclear annihilation of the US).
In the anti-war coalition: finance, entertainment, agriculture. Out: military, manufacturing, intel agencies, news media. Maybe: tech, energy.
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We'll begin examining the Republican sectors, which are material: military, manufacturing, agriculture, and energy. They are mostly pro-war, but there's a chance to peel off agriculture.
The military is out for obvious reasons, and so is manufacturing, since so much of what's left in the US manufacturing sector is war-oriented, and the remainder would be commandeered for such purposes in wartime, being handsomely compensated for their troubles.
There is one Republican sector that is squarely against war, and that is agriculture. Whatever rise in production they would get from feeding a surge in troops would not be a drop in the bucket of their overall business. And that's assuming we had a massive number of boots on the ground -- if fighting were done by drone operators in America, there wouldn't be an increase in food procurement at all.
It would also destroy a potential export market for our crops, not that we send that much to Russia at present, but probably more than we would be sending to US troops deployed over there. Net loss.
And although disrupting Russian society would impair their agricultural output, if they hit us on our own land -- unlike the regional, not global, superpowers we have fought before -- our own agricultural output would get disrupted.
Even assuming their farms suffered while ours did not, our farms would not benefit for long from the boost to prices caused by the drop in global supply. That happened after WWI, when Europe's farms were ravaged while ours were not. US agribusiness went on a leveraged buy-out binge of farmland, assuming that prices would stay elevated because Europe would take so long to recover their farmlands. Unfortunately it only took a few years, and the ag sector back here was devastated for most of the 1920s, crippled by the debt it took on to fuel its M&A, and with no way to pay it off after prices fell back to normal upon Europe's recovery.
Back during the Progressive and New Deal eras, the GOP did not have the South, where the military sector is concentrated, but they did have the Plains states, where agriculture is concentrated (today they have both). In those two eras, the GOP was the isolationist party, and the Dems the interventionists. During the 1976 vice presidential debate, Bob Dole, a Republican from farm state Kansas, griped: "I figured up the other day, if we added up the killed and wounded in Democrat wars in this century, it'd be about one point six million Americans - enough to fill the city of Detroit."
Today, the lone proponents of anti-interventionism among the GOP elites are the much maligned Koch brothers network. They're the ones who anti-war libertarians like Rand Paul, Mike Lee, Justin Amash, and Thomas Massie are working on behalf of in Washington. And they are in the agriculture and energy sectors.
Energy also has little to gain from WWIII against Russia, for the same reasons as agriculture (both do natural resource extraction and sell non-industrial commodities, which would not get sucked into wartime production). Look at how long we've been militarily occupying Japan, South Korea, Germany, Italy, and Afghanistan -- where there's no oil, where we are not wiping out local energy supplies in order to create an artificial market for our own coal, etc. The same would hold for Russia -- nothing to gain, certainly beyond a horizon of a few years.
However, the oil sector is far more likely than other commodity producers to want to wage war against its sectoral rivals overseas (Iran, Iraq), if it cannot co-opt them as client states (Saudi Arabia). And Russia is a major oil and natural gas producer, so they certainly would be in our energy sector's cross-hairs. I put energy into the "maybe" column.
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Now we'll move onto the Democrat sectors, which are informational: finance, tech, media / entertainment, and intel. These are mostly anti-war, with intel and the news sub-sector of the media being exceptions.
The intel agencies are out for obvious reasons, and so are their news media partners. At this point, the two are so intertwined as to form a single propaganda nexus, with a typical panel on MSNBC being at least 50% spooks. Whereas the military and arms manufacturers have the material side of warmongering covered, the intel and news media have the informational side of warmongering covered.
The entertainment industry has nothing to gain from war, and only foreign box office revenues to lose. They are not surprisingly the most outspoken anti-war sector of the elites.
The parts of the tech industry that are like entertainment are equally uninvolved in promoting war. Netflix is entertainment mediated by the internet, and does not rely on news programming in its content. Plus it operates Netflix in Russia, along with most of the world, and would only stand to lose that revenue stream if war broke out.
Amazon is a retailer and service provider mediated by the internet, which could not help out in a war effort. If anything, their international business from Russia would be harmed.
Ditto for Google, who make money from digital ads and monetizing user information -- including users from Russia, who view ads placed in Russia.
Recently both Amazon and Google folded to Russia's demands that they end cloud services for the Telegram app, created by two Russians, which Russia's intel agency FSB was trying to get access to, for monitoring terrorists (and whatever other intel purposes). They do too much business there to take an ideological stand against their host government.
Apple is an electronics manufacturer, so they could get commandeered into the war effort, though it seems unlikely since so much of their stuff is made in China. And the company would not want to jeopardize such a large market for the iPhone, when sales are already saturating for new models.
Facebook makes its money like Google does, but unlike Google for internet searches, it is not the dominant firm in Russian social network sites (that would be VKontakte). Facebook wouldn't gain anything, but they wouldn't lose anything either. And given the pressure they come under here for their role in Russiagate, they are more likely to bend to the warmongering side, albeit on the informational side, joining the propaganda nexus. They're already doing that, genuflecting before Congress about a handful of ads from Russian troll farms that nobody saw before the 2016 election.
That leaves the senior sector of the Democrat side -- finance. War disrupts commerce, financially ruins the home nation from the massive debt it takes on to pay for the war effort (with no spoils won to pay it off, in our war-losing era after WWII), and financially ruins the foreign country, whom our own finance sector may be making loans to or investing in, which they will no longer recoup let alone make a profit on. Our soaring debt, if not rapidly paid off, will lead to either a default or rampant inflation to pay it off -- both scenarios blowing up the assets of the financial institutions.
And they have plenty to lose: JP Morgan and Goldman Sachs were the #2 and #3 investment banks operating in Russia as of April last year. This April, after harsh sanctions imposed by the GOP government on firms doing business with Russia or its oligarchs, global banks have been exiting the Russian financial services market, and any market tied to an oligarch. Goldman Sachs and Citigroup had been advising on the IPO of an Italian company, Octo, whose controlling shareholder is a Russian oligarch targeted by the sanctions (Vekselberg). Whatever fees and commission they were planning on collecting -- poof, up in a puff of smoke because of the anti-Russia hysteria.
In reaction to these sanctions, Russia has liquidated its holdings of US treasuries, from roughly $100 billion to $15 billion, sending the yields skyrocketing on our government bonds for two months straight. As the current decade-long stock market bubble stands ready to pop before election 2020, the last thing the finance sector wants is for the bond market bubble to get popped first -- that is supposed to be a relative safe haven, for investors to pour into when the riskier stocks get blown up. If the safe haven gets destroyed first, it'll wipe out the riskier market by transitivity.
For similar reasons, it was the party of finance who crafted the Iran nuclear deal, so they could earn returns on investment in a large and fairly prosperous nation, which had been closed off to investment by militaristic sanctions for decades. With the finance party out of power, that deal has been scrapped by the return of the militarist party.
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This analysis suggests that the Democrats would be the ideal party to house the anti-war coalition. However, it would have to muzzle or more likely kick out some of its current key members -- the intel agencies and the news media, although not the entertainment media, and perhaps Facebook. These are junior partners to the senior partner of Wall Street banks, so it's do-able. They would have to take in agriculture from the other party to fill the void, perhaps with energy too.
It's also possible that in the lead-up to our Second Civil War, the parallels will be the Democrats going defunct just like the Whigs did (opposition party), to be replaced by a whole new major party (Republicans then, something else now).
When the opposition Whigs morphed into the ascendant Republicans to dethrone the formerly dominant Jacksonian Democrats, they kept most of their old members, but they kicked out those who were too sympathetic to the rival party on the major issue of the day -- slavery, for which the Know-Nothing faction of Whigs felt too much sympathy.
Today, that would take the form of the opposition Democrats morphing into an ascendant anti-war party to dethrone the militarist Reaganite GOP -- but having to kick out its faction that sympathizes too much with ever-rising military spending and interventionism, namely the propaganda nexus of the intel agencies and news media. Let the Republicans have them, and let the anti-war agribusinesses join the Democrats or their successor party.