Sweden and Finland have applied to join NATO.
Finland has an 830-mile border with Russia, so Russia's border with NATO would more than double.
Russia has made repeated threats and said it would have to retaliate if they join.
Sweden and Finland applied on Wednesday to join the NATO military alliance, likely frustrating a long-term aim of President Vladimir Putin to roll NATO borders back.
If accepted, their presence would instantly more than double the border between NATO countries and Russia, from around 750 miles to around 1,600.
At present, NATO has land borders with Russia spanning 754 miles across northern Norway, eastern Latvia and Estonia, and the borders with Poland and Lithuania around Russia's Kaliningrad region.
Finland joining would add 830 miles of continuous border along northwestern Russia, taking the total to 1,584 miles. Sweden has no land border with Russia, but is a major presence along the coast of the Baltic Sea.
The increase in public and political support in Sweden and Finland for joining NATO came after Russia's invasion of Ukraine in February.
Experts have said the development is an own goal for Russia, which partly justified its invasion of Ukraine as a way to stop NATO expanding further.
Russian officials have repeatedly threatened the two countries, saying they would have retaliate against a membership bid. As the application became a reality, Russia moderated its tone.
It could take weeks or months for the two countries to become members.
Turkey has threatened to block their bid, though Finland's president said he thinks that issue will be resolved.
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