It was not long ago that the nation's largest brewers pledged to work together to fend off the liquor and wine industries, which have been stealing market share from the beer industry for years. But any illusion of unity was shattered on Super Bowl Sunday when Aneusher-Busch InBev used advertising's biggest stage to launch a direct attack on MillerCoors.

The marketing assault—in which Bud Light used three expensive ads to criticize Coors Light and Miller Lite for using corn syrup—opened a new front in the beer wars that is likely to resonate for months, if not years. It is also indicative of the depressed state of play for the nation's largest beer brands.

Exactly how MillerCoors responds with its advertising will fall to Michelle St. Jacques, the brewer's new chief marketing officer. The Kraft Heinz veteran's first official day on the job is today, putting her squarely in the middle of the high-stakes marketing battle right from the start. She was unavailable for an interview on Monday. In a post to her LinkedIn page Sunday, she stated, "Monday is a big day - it is day zero. Let's do this."

While AB InBev and MillerCoors still collectively control a massive amount of market share, their largest brands have been in decline for years, as younger generations drink less alcohol and those who do still drink regularly gravitate to smaller craft brands or liquor. That means MillerCoors and AB InBev must duel for pieces of a shrinking pie. So the only way to grow is to steal market share from each other.

"The category is declining no matter what you do," says one ad industry exec who has previously done work for AB InBev. "They will end up fighting for the table stakes."

The Bud Light ads by Wieden & Kennedy New York dramatized its competitors use of corn syrup by showing big barrels of it delivered to mythical Coors Light and Miller Lite kingdoms as part of Bud Light's ongoing medieval-themed campaign.

It did not take long for MillerCoors to respond. "The Super Bowl just ended, but the contest between our products and Bud Light is far from over," MillerCoors CEO Gavin Hattersley wrote in an email to distributors Sunday night. "We're proud of our beers, we're proud of our ingredients and we're happy to have this fight any day of the week."

MillerCoors actually started the fight with a campaign Miller Lite began in 2016 that claims Lite has "more taste and half the carbs" than Bud Light. (Miller Lite has 3.2 grams of carbs per 12-ounce serving and 96 calories. Bud Light has 6.6 grams of carbs and 110 calories.)

But Bud Light's attack is unusual because it is rare to see the bigger brand call out its smaller competitors. (Coke barely acknowledges Pepsi's existence, for instance, and the same can be said for McDonald's when it comes to Burger King.) Therefore, people inside MillerCoors are bullish that Bud Light's attack could play into their hands. "In some ways they gave us a gift," Pete Marino, MillerCoors' chief public affairs and communications officer, told Ad Age. "Our guys are competitive. They grew up fighting Anheuser-Busch."

While Bud Light's ads made pointed attacks, brand VP Andy Goeler in an interview Sunday afternoon pivoted away from directly calling out his competitor, routinely making an argument that Bud Light—which recently put big ingredient labels on its packaging—was simply making a call for transparency. "As the lead brand in the industry this is something that's good for beer. It's good for us, the lead brand, to make a bold move like this," he said.

For now, MillerCoors has confined its counter-attack to its corporate Twitter handle, which on Sunday made the distinction between high fructose corn syrup—which it says AB InBev uses in some of its products— and corn syrup.

At MillerCoors, we're proud of our high-quality, great-tasting beers. We're also proud that none of our products include any high fructose corn syrup, while a number of Anheuser-Busch products do. And Miller Lite has fewer calories, fewer carbs and more taste than Bud Light. pic.twitter.com/GeUUXqnSpc

— MillerCoors (@MillerCoors) February 4, 2019

In his memo to distributors, Hattersley defended MillerCoors' use of corn syrup by saying it is used in the fermentation process for Miller Lite and Coors Light, but none of it makes it into the final liquid that is sold. The brewer on Monday gained an ally in Washington Post food/science columnist Tamar Haspel, who sent several Tweets digging into the weeds of beer-making, while concluding that Bud Light's ads amounted to fear-mongering.

Sometimes brewers want to up the alcohol without adding color, flavor, or aroma, so they may turn to CORN SYRUP. It is also CHEAP! @BudLight, which started this fight, uses RICE (also CHEAP) for the same purpose. First, they cook it so the sugars are usable by the yeast.

— Tamar Haspel (@TamarHaspel) February 4, 2019

In a statement Monday responding to the criticism, AB InBev said, "consumers are demanding more and more transparency. Bud Light's Super Bowl commercials are only meant to point out a key difference in Bud Light from some other light beers. This effort is to provide consumers transparency and elevate the beer category."

The brewer also acknowledged that "yes, some of our products do use corn syrup. You can find the ingredients for our brands at TapIntoYourbeer.com. Different beers have different recipes that create different flavors. Consumers want transparency and we're providing it. It's up to them to decide what beer is right for them."

According to the site, AB InBev brands that use high-fructose corn syrup include Lime-A-Rita and Best Damn Root Beer. Brands that use regular corn syrup include Bon & Viv Spiked Seltzer, one of the brands AB InBev plugged Sunday with a Super Bowl ad.