Unfortunately, despite the fact of they have said that they will never use them, a good number of states still possess or are believed to possess nuclear weapons: the United States, Russia, the United Kingdom, France, China, India, Pakistan, Israel and North Korea. And many others might also have it, just better hidden. So the possibility of a nuclear incident happening any time still exists, either caused by an armed conflict, such as the Syrian War , a terrorist attack or even by mistake or miscalculation.
The United States
The United States is the only country in the world which has used nuclear weapons in combat and it was the first one to manufacture them. Despite the fact that the country has signed the treaty, it is believed that they maintain an arsenal of 4,760 warheads - it is supposed that 1,950 of them are strategically operational - and facilities where they can design and build many more warheads.
Also, it has been recently revealed plans to spend a trillion of dollars in the modernization of their nuclear arsenal. The reason used to explain it was the fact that maintenance had become necessary, but there are rumours saying that this an answer from the United States to the increasing number of terrorist attacks in the world and possible plans for more nuclear weapons in Russia and North Korea.
It is believed that Russia possess around 8,500 nuclear warheads and that 1,800 of them are strategically operational. They have also signed the treaty, but, following the same understanding of other countries, they defend that is important to keep part of their arsenal active in case it becomes necessary in response to the use of weapons of mass destructions against them or their allies.
Russia has also been accused of supporting North Korea's nuclear program, but they have fiercely denied it. And recently the international news was also flooded by their possible plans of building a nuclear device codenamed Status-6, able to create an extensive zone of radioactive contamination along the enemy's coast so to make it uninhabitable for a long time.
And North Korea has spent more and more of their budget on manufacturing warheads under the allegation of trying to protect themselves against constant threats from the United States. They have withdrawn their participation in the treaty and might have fewer than 10 warheads currently -- but it isn't known if they have the means to deliver them.
They also claimed to have conducted an H-bomb test, although weaponry specialists declared that the measurements of seismic disturbances indicated that the detonation wasn't consistent with a hydrogen bomb.
While other countries possess a smaller number of warheads, they still can be considered as an international treat. The United Kingdom seems to have 215 warheads and maintain a fleet of four nuclear-armed submarines in Scotland. France also has deployed most of their nuclear weapons, believed to be 300 warheads, on submarines, while others are deliverable by aircraft.
China, India and Pakistan has been recently increasing the size of their nuclear arsenal. They are supposed to have 260, 120, and 130 warheads, respectively. Israel has never confirmed or denied the existence of a national nuclear arsenal, but it seems that they own around 80 warheads. Plus, Belgium, Germany, Italy, Netherlands, and Turkey are believed to host nuclear weapons for other countries.
While most of the people expect that nobody will ever press the button, it is impossible to be sure that it will never happen. It seems clear to everybody that starting a nuclear war wouldn't be wise to anyone, as the proportion of the disasters would be even more catastrophic compared to what the world faced during the Second World War.