Writing on X, Colby said the review aims to ensure NATO moves "fast and irreversibly" towards Europe taking the lead on its own conventional defence.
He described it as part of a push to transform the alliance into a "stronger, more equitable, and sustainable" partnership – what he called "NATO 3.0" – restoring "its character as a powerful, war-fighting organisation."
Colby said the review would set out options grounded in strategic rigour and a clear-eyed reading of current geopolitical and military realities, aligned with "the 2025 National Security Strategy and the 2026 National Defense Strategy" – core US strategy documents.
He said he expected to work closely with NATO's Supreme Allied Commander Europe, General Alexus Grynkewich, and to keep consulting allies, Congress and other parts of the US government.
Defence Secretary Pete Hegseth first announced the review in Brussels in June, saying it would take up to six months and that some NATO members would "pass" the test while others would "fail" it.
The Wall Street Journal reported in early July that Hegseth and Colby had been pushing particularly hard to cut US force numbers in Europe.
Colby has long argued for scaling back American commitments outside Asia to free up resources for countering China.
Poland, however, has been given assurances.
Colby said in early July, after meeting Poland's national security chief Bartosz Grodecki, that talks had focused on strengthening the US presence in the country.
Grodecki told reporters a paused troop rotation would be "completed" and that the Pentagon was open to a permanent US base in Poland.
Last week, Poland's Deputy Defence Minister Magdalena Sobkowiak-Czarnecka said US officials had told her that while some withdrawals from Europe are, and likely will be, happening, the country would not be affected.
(ał)
Source: PAP