First use of a nuclear weapon against the US...

will not be against the US mainland (by a foreign power as opposed to terrorist organization).   Rather it will be used at sea against a carrier task force.  This article seems to indicate a conventional weapon but I will bet my licence on the fact it is capable of a nuclear payload.

Short extract:

Translated by the naval affairs blog Information Dissemination, a recent report provides a description of an anti-ship ballistic missile (ASBM) that can strike carriers and other U.S. vessels at a range of 2000km.

The range of the modified Dong Feng 21 missile is significant in that it covers the areas that are likely hot zones for future confrontations between U.S. and Chinese surface forces.

The size of the missile enables it to carry a warhead big enough to inflict significant damage on a large vessel, providing the Chinese the capability of destroying a U.S. supercarrier in one strike.

Because the missile employs a complex guidance system, low radar signature and a maneuverability that makes its flight path unpredictable, the odds that it can evade tracking systems to reach its target are increased. It is estimated that the missile can travel at mach 10 and reach its maximum range of 2000km in less than 12 minutes.

The US Carrier task force is an unparalleled tool of state power.  It can go virtually anywhere on very short notice and deliver a serious blow to any modern state.  US air superiority is such that almost no nation can stand against even a single attack by our aircraft and standing against sustained attack is nigh impossible.  If you take out the carrier then you can by and large negate that advantage.  This makes a carrier battle group a high value target in a war against a great power like China or Russia.  First of all they are incredibly expensive to build  and operate (can't find the link now but something on the order of $1.5 Billion a year for the whole Battle Group).  This is an enormous sum of money.  Not only that it takes a long time to build capital ships of any sort and a carrier even more so, since we have limited number of ships available (12 or so) they are incredibly tempting targets.  Since  a nuclear strike can take out the whole group fast, with little chance of civilian casualties in a distant location with little or no physical evidence of the misdeed it is essentially a no lose situation for someone in a major war.   In the atmosphere these days what President is going to risk a general nuclear exchange?   This guy?  Doubtful.  So there you have it.  This is why a nuclear first strike against US naval assets is inevitable when we get into the next major war.