• The U.K. is the leader of NATO's presence at this military base and it has contributed with the highest number of soldiers — currently 1,600 troops.
• Colonel Dai Bevan, commander of NATO's Enhanced Forward Presence in Estonia, told CNBC that their day-to-day involves constant training.
• NATO has so far refused to send soldiers to Ukraine to avoid an escalation of the conflict with Russia, fearing that doing so would trigger a new world war.

TAPA, Estonia — Just 70 miles away from the Russian border and an hour's drive west of Estonia's capital, thousands of NATO troops are on alert in case President Vladimir Putin decides to attack.

The NATO military alliance has stepped up its presence at the Tapa military base in central Estonia since Russia first illegally annexed Crimea back in 2014. Geopolitical tensions have become even more apparent since early February as Moscow deployed about 100,000 troops closer to Ukraine — ahead of invading the country later that month.

"Looking at that situation in Ukraine, [it] gives us greater focus and really gives us clarity on the importance of what we do here," Simon Worth, commanding officer of the U.K.'s Royal Tank Regiment, told CNBC.

The U.K. is the leader of NATO's presence at this military base and it has contributed with the highest number of soldiers — currently 1,600 troops.

"It's a defensive presence here, but we thought it would be important to reassure our Estonian partners," Ross Allen, U.K. Ambassador to Estonia, told CNBC about Britain's decision to have more troops in the country.

He added that the U.K. could decide to extend the current presence of these troops in the region.

Denmark and France have also sent soldiers to this base, about 200 each. In addition, 2,000 Estonian soldiers are permanently here — bringing a total of 4,000 troops on the ground.

It means that the Tapa military base is now at full capacity, with some of the soldiers having to sleep in tents.

Colonel Dai Bevan, commander of NATO's Enhanced Forward Presence in Estonia, told CNBC that their day-to-day involves constant training.

"To make sure we are ready to respond to any adversary that threatens either Estonia or the wider NATO area," he said.

Defense and military officials in Estonia do not think that Russia is due to attack any NATO country, partly because Russian troops have struggled to make quick advancements in Ukraine. However, the defense alliance is on high alert, particularly over potential cyber attacks as well as misinformation.

NATO has so far refused to send soldiers to Ukraine to avoid an escalation of the conflict with Russia, fearing that doing so would trigger a new world war. But it has nonetheless sent equipment to Ukrainian forces while also reinforcing its presence across all eastern Europe.

The latest data shows that Poland hosts the highest number of NATO troops, at 10,500. There, NATO's battlegroup is led by the United States.

The defense alliance has also sent more planes and ships to eastern Europe and has said it could do more if needed.

Speaking ahead of an extraordinary NATO leaders summit on Thursday, Secretary-General Jens Stoltenberg said the group is likely to send new troops to Bulgaria, Hungary, Romania and Slovakia.

"I expect leaders will agree to strengthen NATO's posture in all domains, with major increases in the eastern part of the alliance on land, in the air and at sea," Stoltenberg told reporters Wednesday.