Hostilities have now broken out with Tencent.
The two had co-existed happily: since richer Chinese prefer iPhones to Android phones, these devices are where WeChat made much of its money.
But earlier this year, WeChat launched “mini-programmes,” a form of lightweight app that operates independently of Apple's app store and robs it of revenues.
Apple, meanwhile, had disliked but tolerated WeChat's practice of allowing users to reward generators of content (for example, opinion columns) with small tips.
These bypass Apple's own payments mechanism.
On April 19th Apple obliged WeChat to shut down tipping.
Another front in the fighting is that the American firm's mainland app store accepts Alipay, a payment service from China's Alibaba, but not WeChat's payment offering.
Broadly, WeChat is going from being a social-media platform (akin to Facebook and WhatsApp rolled into one) to becoming a mobile-operating system, putting it on a collision course with Apple.
“There is a war going on,” says Mr Brennan.
Who will win such a clash of titans?
Rumours are swirling among tech experts about what might happen next.
Apple is trying to fortify its position.
It is investing heavily in its large network of stores and research labs on the mainland; and it plans to include China in the first wave of countries in which its highly anticipated new iPhone will be launched later this year.
But Apple is on the defensive, whereas Tencent is firmly on the attack.
Mr Brennan speculates that Tencent might even launch a WeChat phone, which would make Tencent's offering completely independent of the iPhone.
Anywhere else in the world, it would be foolish to go up against the Californian giant.
In China, though, the native firm may have the advantage.
As Connie Chan of Andreessen Horowitz, an investment fund in Silicon Valley, puts it: “Loyalty is much, much stronger to WeChat than to Apple in China.”
硅谷的Andreessen Horowitz（一家风险投资基金公司）的Connie Chan 讲到：在中国，用户对于微信的忠诚要比苹果强的多。