Apple and Ireland were Tuesday quick to announce plans to appeal the European Commission’s $14.5 billion tax ruling against the tech giant, but they shouldn’t forget who they are up against.

Margrethe Vestager, the EU’s hard-hitting competition commissioner, also known as “the Iron Lady of Denmark” for taking on corporate giants in antitrust cases, is determined in upholding the state-aid decision on Apple AAPL, +3.03% , remaining confident the case can stand up to any appeal.

“We have of course structured the case in a way that it’ll be upheld if it goes to court. That’s why it sometimes takes a long time to finish these investigations,” she said in an interview with MarketWatch.

“This probe has been three years under way to make sure we really have a solid case, which I firmly believe we do,” she added.

The EU commission ruled on Tuesday that Apple’s tax arrangement with Ireland constituted illegal state-aid, ordering the Irish government to recover as much as 13 billion euros ($14.5 billion) in taxes from the iPhone maker. The decision prompted criticism from many corners, with critics arguing the ruling will make Europe less attractive for foreign investment and that Apple hadn’t done anything wrong.

Apple CEO Tim Cook, for example, said in a letter to customers the move has “serious, wide-reaching implications” and strikes a “devastating blow to the sovereignty of EU member states over their own tax matters.”

Similarly, Irish finance minister Michael Noonan said he “profoundly” disagrees with the commission, stressing it’s “important that we send a strong message that Ireland remains an attractive and stable location of choice for long-term substantive investment.”

Even the U.S. Treasury and White House got involved. A spokesperson said the Treasury was “disappointed” with the EU’s decision and that it could threaten “the important spirit of economic partnership between the U.S. and the EU.”

Read:U.S. taxpayers would be poorer if EU wins Apple tax case, White House spokesman says

But Vestager was unfazed.

“If that’s how the Treasury feels, then that’s their experience and we can’t take that away from them. All I can say is that we didn’t open this case to upset you,” she said. “ We did it to make sure we have fair competition on Europe.”

The EU’s antitrust investigation concluded that almost all Apple profits recorded by the company’s Irish incorporated entities were internally transferred to a so-called head office that existed only on paper. That arrangement allowed Apple to effectively only pay 1% in taxes on its European profits in 2003 and as little as 0.005% in 2014, way below Ireland’s corporate tax rate of 12.5%.

“Ireland already has a very attractive corporate tax regime compared to other European countries, so it’s hard for me to understand that it’s fair to allow individual companies to receive state aid in the form of lower taxes,” Vestager said.

EU’s Apple tax ruling turns spotlight on multinationals

Apple’s Irish tax deal also meant the company avoided taxation on almost all profits from sales of its products in the EU single market, as the sales were recorded in Ireland rather than in the country where the transaction took place. That structure doesn’t fall under the state-aid oversight, but Vestager hopes this case will prompt EU member states to revisit their own tax rules and review how they treated Apple.

If a country finds Apple should have paid taxes where their products were sold, they can now recover it under Tuesday’s ruling. This would be subtracted from the up to €13 billion to be recovered from Ireland.

But Vestager isn’t done in her sweeping inquiry into sweetheart tax deals in the EU. In Tuesday’s press conference she reminded that the commission is still investigating Amazon.com Inc. AMZN, +0.18% and McDonald’s Corp. MCD, -1.75% for their tax agreements with Luxembourg.

“We are investigating other issues than in the Apple case, but it’s still about taxes,” she told MarketWatch. Neither Amazon nor McDonald’s have responded to the ongoing probes, she said.