Combined International Corporation forms Apr 1982 under Patrick Ryan (Ryan Insurance) merging with W. Clement Stone's Combined Insurance of America; renamed Aon (Gaelic for 'oneness') 1987. Ryan's vision: build a global insurance brokerage to rival Marsh & McLennan via aggressive M&A.

Combined International officially renamed Aon Corp 1987 — Gaelic word for 'oneness'. Patrick Ryan's vision: a unified global insurance services firm. Aon goes public 1988 via Combined International; aggressive M&A through 1990s acquires Alexander & Alexander (1997, $1.2B), Bain Hogg, Hudig-Langeveldt.

Aon acquires Alexander & Alexander 1997 for $1.2B — combines #4 + #5 US brokers; becomes #2 global broker behind Marsh. A&A had earlier acquired Sphere Drake. Patrick Ryan's aggressive M&A continues through 1990s consolidating fragmented US/UK brokerage market.

Greg Case becomes Aon CEO Apr 2005 (under Patrick Ryan transition) — McKinsey + Citi background. Case redefines Aon as 'global professional services firm' beyond brokerage. Aon stock 6x's 2005-2025 under Case as broker fee model + global reach compound.

Aon acquires Hewitt Associates Oct 2010 for $4.9B — adds HR consulting + employee benefits administration. Aon Hewitt becomes top-3 global benefits consultancy. Aon further diversifies from pure brokerage; expands services revenue + recurring fees.

Aon executes corporate inversion Apr 2012 — redomiciles from US to UK to lower tax rate; HQ moves Chicago → London. Becomes one of the largest US tax-inversion controversies pre-TCJA. Aon later moves UK → Ireland Apr 2018, then back to US 2020 after Trump tax reform makes US domicile attractive again.

Aon redomiciles UK → Republic of Ireland Apr 2018 — corporate inversion to access Ireland's lower 12.5% corporate tax rate. Becomes one of largest US-listed Irish-domiciled companies. Returns to US HQ Chicago 2020 post-TCJA when US corporate tax dropped to 21% making Ireland less advantageous.

Aon Marsh & McLennan rumored mega-merger talks Mar 2019 — discussions of combining #1 + #2 global insurance brokers (would have created $20B+ revenue platform). Aon abandons within 24 hours after media leak. Greg Case CEO later pursues Willis Towers Watson instead 2020.

Aon announces $30B all-stock merger with Willis Towers Watson Mar 9 2020 — proposed combination of #2 + #3 global brokers. DOJ antitrust lawsuit Jun 16 2021 + abandonment Jul 26 2021. Aon pays $1B reverse termination fee to WTW. Major setback but Aon stock recovers; NFP acquired Apr 2024 fills mid-market gap.

Aon-Willis Towers Watson $30B merger abandoned Jul 2021 — DOJ antitrust lawsuit Jun 16 2021 alleges combination would harm large-company benefits + insurance brokerage competition. Aon pays $1B reverse termination fee. WTW remains independent. Setback for industry consolidation thesis among 'Big 3' brokers.

Greg Case CEO of Aon delivers 2021 'Innovation in Risk Capital' message Sep 2021 — outlines pivot toward broking pension transactions, intangible asset risk, cyber risk transfer. Aon stock 3x's 2020-2023 even after WTW failure as broker fee model proves resilient through pandemic + hardening.

Aon acquires NFP Apr 1 2024 for $13.4B from Madison Dearborn Partners — NFP is top-10 US mid-market commercial brokerage + benefits/wealth. Adds 7,700 employees + 600+ offices to Aon's enterprise. Aon's largest acquisition; fills gap in US mid-market that competitor AJG dominated.

Aon closes NFP acquisition Apr 1 2024 for $13.4B cash + stock — buys US mid-market commercial brokerage + benefits + wealth from Madison Dearborn. Adds 7,700 employees + 600+ offices. Aon's largest acquisition; fills mid-market gap competitor AJG dominated. Greg Case CEO executes deal post-WTW failure.

Aon Q4 2024 — Greg Case CEO reports $14.4B revenue 2024 (+12% YoY); NFP acquisition (Apr 2024 $13.4B) integrating well. Aon stock $400+ all-time high. Aon's tech investment + tax-efficient Irish HQ + pure broker model continues to compound; double-digit annual EPS growth maintained.