It is based on a claim – quite possibly true – that Russia is moving an Iskander missile system by sea to their Baltic Sea enclave of Kalingrad. The article then quotes various Estonian officials, including the Prime Minister, who paint this as an aggressive action by Russia. For example:
“In any case, what is called for now is to remain calm, and to treat these incidents as attempted blackmail,” Mihkelson said. “Russia is simply showing its desire to reinforce its position at the entrance to the Baltic Sea.”
[Estonian member of parliament and member of a defense committee]
References to Iskander missile system being transported by the Baltic Sea to Kaliningrad are certainly alarming and show yet again Russia’s attempts to pressure the west by using different tools.
[Taavi Rõivas, Estonian Prime Minister]
This is a nice example of the ludicrous nature of these claims about an “aggressive Russia”. The Guardian also quotes an unnamed “Estonian defense expert” as saying: “It can carry nuclear weapons, change direction mid-flight and fly distances of up to 500km. As such it is capable of threatening Poland, including the US missile defense installations there.” And there is the rub. The US has placed a system in Poland which a) could potentially shoot down nuclear ICBMs launched from Russia (thus undermining the potency of the Russian nuclear deterrent) and b) can, at least according to the Russians, be easily converted to fire short-range Tomahawk missiles at Russia.  One variant of the Tomahawk missile can carry a nuclear war-head.  (This variant is reported by Wikileaks as retired – but obviously the system has such a capacity at least potentially). You don’t, surely, have to be a military expert to see that Russia might well want to respond to such a development with a military counter-measure? Indeed they have been warning for some time that they might do exactly this.  This then is a classic example of how the West builds the narrative about an “aggressive Russia”. In reality the Russian actions are an obvious military counter-measure to a military move by the US. But their actions are presented (in this case by NATO member Estonia and amplified by the Guardian) as being a military move made without foundation; sinister, underhand, aggressive and so on.
The fact that Russia has for some time been publicly mulling whether or not to place Iskanders in Kalingrad  undermines one of the main arguments of the Estonian officials – that this has been done in ‘secret’. – It may be that the weapons are being shipped in ‘under cover’; that would be a normal military precaution – but their arrival is a matter which the Russians have quite openly discussed.
The Guardian also explains, that:
The Iskander-M, the Persian name for Alexander the Great, is a ballistic rocket system designed to destroy strategic targets, and its stationing is arguably in breach of the intermediate-range nuclear forces treaty
However. The Russians also argue that the US missile system which the Iskander is intended to counter breaches this treaty.  By only mentioning one side the Guardian presents a one-sided picture in which Russia is seen as the one in breach of treaties etc. (The range of the Iskander system is up to 500km. It is specifically designed to be within the limits of the intermediate range nuclear forces treaty  which bans land-based missiles with a range between 500km and 5000km.  Russia does have a missile which is arguably in breach of this treaty, the R-500. This missile uses the Iskander launcher.  If Russia is said to be in breach of the INF treaty by virtue of deploying an Iskander system which has the potential to fire a certain kind of missile then by virtue of the same argument the US is in breach of the same treaty – by deploying a system which can also be adapted to fire a medium-range land-based cruise missile).
The Guardian article also adds:
On Thursday, a Russian military An-72 aircraft penetrated Estonian airspace over the island of Vaindloo without permission and spent about minute and a half in the country’s airspace.
The aircraft transponder was switched on, but no flight plan was submitted and the aircraft did not respond to radio contact with the Estonian air movement service
The source for this (not given) is presumably the Estonians or Finns. Russia has denied the claim.  The Guardian is not obliged to report this but it would make its reporting look less one-sided.
This is a characteristic piece of Guardian anti-Russia propaganda. It reports the obviously unbalanced claims by NATO member Estonia concerning the (claimed) deployment of the Iskander system to Kalingrad. The presentation of this as an aggressive action is clearly fallacious. Three separate Estonian officials are quoted as well as an unnamed “defense expert”. The Guardian then embellishes the article with some more one-sided claims and explanations. The other side of the picture is completely absent. This is how war propaganda is written, not news.
Here is the follow-up article in the Guardian. It is even more hyperbolic than the original one. The headline “Russia transfers nuclear-capable missiles to Kalingrad” is completely misleading. The point about the Iskander missile system is that it is a very accurate short-range (500km) land based ‘Scud’ type missile. Its role is to eliminate enemy rocket launchers, command posts etc. The military reason why the Russians may have transferred an Iskander system to Kalingrad would be to oppose the threat to them posed by the planned US missile system in Poland. (Even the unnamed ‘defense expert’ quoted in their original article acknowledged this). To present this as a piece of nuclear aggression on the part of Russia is horribly irresponsible. It does nothing but whip up false fears. It is also bad journalism since it is simply wrong. As before the Guardian quotes the US side (in the form of an unnamed intelligence official) who presents the US/NATO propaganda line on this – that it is a “gesture” on the part of Russia. This is disinformation to disguise the real military situation – Russia is/may be taking a logical military step in relation to a concrete military threat on their borders. Again; to amplify US disinformation as if it were news has nothing to do with journalism.