Retired Lt. Gen. Mark Hertling is a national security, intelligence and terrorism analyst for CNN. He served for 37 years in the Army, including three years in combat, and retired as commanding general of US Army Europe and 7th Army. He is the author of "Growing Physician Leaders." The opinions expressed in this commentary are solely his.

(CNN) In 1975, the Cold War was at its peak. I was a new tank lieutenant starting my career with a three-year assignment in US Army Europe. There were 250,000 US soldiers on the continent, and our job was patrolling the border fences between West Germany and the Warsaw Pact nations and defending Europe from Soviet aggression.

Today the Soviet Union is no more, and the United States has only about a 10th of the permanent fighting force in Europe that it had 42 years ago. Still, the mission of defending against aggression remains a priority -- as is being made clear this weekend by the arrival of the first rotating US brigade in Poland

This is not something that was recently planned. This is, in effect, the final act of a massive, decade-plus transformation of US force presence in Europe. And whether or not the new Trump administration supports the continued rotations as a counter to Vladimir Putin's expansionist approach is a key question even as the first brigade begins its nine-month tour in Eastern Europe.

The Berlin Wall comes down

In the middle of my second tour in Europe -- November 1989 -- I was a major and on "border duty" with a cavalry squadron near the West German-Czechoslovakian border. After months of political upheaval, we heard on the radio of the decision to allow unchecked civilian traffic between East and West. That action precipitated a flow of East Germans to freedom and it couldn't be stopped; within months, the Berlin Wall came down and the various countries of the Soviet bloc began breaking away from the Soviet Union.

Soon, we were without an official mission ... and the decision makers in Washington issued the orders for the beginning of a drawdown of forces as a result of the peace dividends associated with winning the Cold War. Had it not been for Saddam Hussein's invasion of Kuwait in 1990, the force reduction in Europe and the associated return of forces to the United States would have been realized much sooner than later.

But even with the force drawdown delay caused by the war in Iraq (after all, more than 60,000 soldiers from Europe deployed to the sands of Saudi Arabia and Kuwait during Operation Desert Storm), the size of the US Army in Europe still began decreasing from a quarter million to about 90,000 soldiers by the end of 1998. US Army Europe would stabilize, but there were still two armored divisions (about 20,000 soldiers, with lots of tanks and related combat power) and a corps headquarters.

Start of a long war

Forward to 2001. I had just been assigned to the Pentagon when a plane hit the building. We were at the start of what we did not yet know would be a long war. But while US forces were deploying to Afghanistan and a few months short of the US invasion of Iraq, Defense Secretary Donald Rumsfeld issued orders to the commanding general of US Army Europe to begin reduction of his force. A number drove the goal: no more than 24,000 soldiers on the continent. Gen. Burwell Baxter "B.B." Bell had the task of drawing down the force, closing most of the bases (called "kasernes" from the German word for "barracks"), and redesigning the "footprint" for the Army in Europe.

In 2003, I left the Pentagon and joined the 1st Armored Division, a unit based in Europe but that had deployed to conduct the attack on Baghdad. After 15 months in combat, our division returned to Germany from Iraq, and I transferred to become the commander of the Army's European Training Center at Grafenwoehr, Germany.

The Army in Europe was in the early throes of transformation to the force designed by Bell, and my job was to train the remaining force, engage and train allied armies, and transform a training center that had been there since 1945.

At a Land Combat Exposition in Washington in September 2004, Bell outlined the design of the new force to the rest of the Army and to the press in Washington. Since Bell was dual-hatted as commander of NATO Allied Forces Central Europe, he was concerned the force he was ordered to build would not have the number of soldiers needed to contribute both to NATO missions and engage with and help train new Eastern European armies.

New way to defend the alliances

Bell presented an initiative he called Joint Task Force-East, or JTF-East. The idea was to rotate, rather than permanently assign, a brigade from the United States through two facilities in Eastern Europe; part of the brigade would be in Romania and the other part in Bulgaria. That rotational brigade -- a force of about 4,000 soldiers with 90 tanks, 140 Bradley infantry fighting vehicles, 18 howitzers, 400 other tracked vehicles, more than 900 trucks and Humvees -- would come to Europe for about nine months, leaving their families and homes in the United States, and at the end of the rotation they would immediately be replaced by another rotational brigade.

They would be temporarily "stationed" within chosen countries, and from there would travel to and conduct training with myriad European allies. This would supplement the end-state European force of 24,000, and the brigade would show the nations who were our NATO partners and our non-NATO allies that we were still defending the alliances.

The overall transformation design was approved, and I moved from the training center to be Bell's G3, the equivalent of a chief of operations to his role as commander or chief executive officer. Bell's job was to drive change; my job was to help him execute it.

The transformation efforts were not done in a vacuum. The plan's timeline was constantly interrupted by many things: the requirement for deploying forces from Europe to Iraq and Afghanistan, the building of new facilities, the consolidation of force at new locations, the bureaucracies associated with both the US and German government approvals. But while often delayed, the transformation continued.

In 2007, Bell and I moved to other jobs: He went to command forces in Korea, and I went to command the 1st Armored Division in northern Iraq. We both watched from afar as the transformation of Europe continued. We both wondered whether a rotational brigade would ever deploy to JTF-East.

In 2011, I was selected to command US Army Europe. It was a dream job. The command was in its final stages of transformation; the number of soldiers assigned would be closer to 33,000 rather than the 24,000 foreseen during the Bush administration; our units were still training for and participating in combat in Iraq and Afghanistan; and we were actively engaging with the armies of 51 countries on the European continent. Amid all this, I began reporting to my bosses intelligence showing increased terrorist ratlines throughout Europe as well as the increased adventurism of Russia's Putin.

We were still uncertain about whether or not we would see a rotational brigade return to Europe when we opened the new headquarters in Wiesbaden, Germany, and then put the last US Abrams tank on a boat back to the United States. Both those actions marked the end of transformation. There were still no indicators the Army and Department of Defense would place an end-to-end rotational brigade in Europe.

Arrival of the brigade

Russian adventurism in Ukraine, Putin's threat toward the Baltics and Poland, and an increased awareness by the Defense Department and Congress regarding the need for an increased US force presence in Europe has changed all that.

On Saturday, the 3d Brigade Combat Team of the 4th Infantry Division from Fort Carson, Colorado, participated in the welcoming ceremony for Operation Atlantic Resolve in Zagan, Poland. Now part of something called the European Reassurance Initiative, the brigade is scheduled to train and engage with armies from Romania, Bulgaria, Hungary, Estonia, Latvia, Lithuania and Poland during its nine-month deployment.

While long in the planning stage, the physical presence of this small force is welcomed by all the NATO countries as it reassures them of continued US support of the alliance. One soldier once told me "when tanks are on the ground, America means business." Hopefully, Russia will understand this small brigade combat team from Colorado means commitment to an ideal that has been in Europe for more than 70 years.