It's no secret where the F-35 is intended to operate - in the highly-defended airspace of just two countries, China and Russia. The F-35 and its big brother the F-22 were designed to defeat sophisticated air defences, at least what would have passed for sophisticated at the turn of the century.
It's hard to fault the intended targets of America's stealth armada for responding to what, for them, is a potentially catastrophic threat. And so they have.
They had a look, several looks, at what Lockheed and its sub-contractors were up to. It's believed they hacked a lot of computers, stole a lot of code and design secrets. Then the Americans made the incredibly stupid mistake of allowing a state-of-the-art stealth drone, the RQ-170, fall into Iranian hands almost intact.
The "bad guys" know more about America's stealth technology than America's allies do and probably ever will. That's not a particularly good thing - for us, at least. They figured out a lot of the weaknesses and vulnerabilities in what are now America's "venerable" stealth warplanes. That has allowed them to develop some impressive air defence systems - multi- and broadband radars, long distance electro-optical sensors, multi-sensor anti-aircraft missiles. They've also fielded their own stealth warplanes.
Here's the Russian entry, the Sukhoi PAK-FA. This is designed to give the F-22 all it can handle. It could also give the F-35 absolute fits.
Like the F-22, the Sukhoi is long-range, very fast and features thrust-vectoring nozzles. Unlike the F-22, the Sukhoi's engines are set well apart to enhance control inputs and the nozzles can rotate through 360 degrees. At the 5:30 mark in the video the Sukhoi appears to demonstrate a completely controlled flat spin.
Another feature of the Russian plane is that it appears to have a combination leading-edge extension/canard which probably remains fixed in stealth mode but comes into operation for close in air-to-air combat.
And, lest we be tempted to berate the Russians for stealing everything the Americans invented, it was a Russian mathematician who devised the calculations for stealth cloaking - all the ratios and angles, etc. Some clever American engineers stumbled across the Russian paper, had it translated and turned it into this (which is Russian for "math"):