English Bus/Financial Law: Taxation & Customs Formation of SE

Hi everybody,
I would like to understand the meaning of "but for" in the following:

"Where tax would, but for the Mergers Directive, have been chargeable in the member State in which the permanent establishment is located, Part 18 of the Taxes Act 1988 (double taxation relief), including any arrangements having effect by virtue of section 788 (double taxation agreements), shall have effect as if the amount of tax that would, but for the Mergers Directive, have been charged in respect of the transfer of the chargeable intangible assets, had actually been charged."

Thank you very much